Use Chrome’s DevTools Protocol (CDP) to emulate prefers-color-scheme: dark before loading the page, then call Selenium’s screenshot method. The emulation changes the page’s media-query preference; Selenium still performs the actual capture. If you change the preference after navigation, reload before taking the screenshot.
What the workflow does
A dark screenshot requires two separate operations:
- Tell Chromium that the user prefers a dark color scheme with
Emulation.setEmulatedMedia. - Capture the rendered page with Selenium, such as
save_screenshot()or an element’sscreenshot()method.
This procedure activates a site’s own dark theme when it responds to the CSS prefers-color-scheme media feature. It does not automatically force every page to darken.
Prerequisites and version notes
- Python 3 and an installed Selenium package.
- Google Chrome or another Chromium browser that matches the driver Selenium starts.
- A page that has a dark style tied to
prefers-color-scheme, or a known selector/style you can use to verify the result.
Install Selenium in the environment that will run the script:
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python -m pip install -U seleniumSelenium’s CDP integration is Chromium-specific and version-sensitive. Selenium documents CDP as a temporary bridge while WebDriver BiDi is implemented, and warns that CDP functionality depends heavily on the browser version. Pin and test compatible browser and Selenium versions in continuous-integration jobs rather than assuming that a command that worked last year will remain unchanged.
Complete Python example
The following script applies the emulation before navigation, captures the page, and always closes the browser:
from selenium import webdriver
options = webdriver.ChromeOptions()
# Uncomment for a server without a visible desktop:
# options.add_argument("--headless=new")
driver = webdriver.Chrome(options=options)
try:
driver.execute_cdp_cmd(
"Emulation.setEmulatedMedia",
{
"features": [
{"name": "prefers-color-scheme", "value": "dark"}
]
},
)
driver.get("https://example.com")
driver.save_screenshot("dark-mode.png")
finally:
driver.quit()The call to execute_cdp_cmd is Selenium’s Python binding for sending a CDP command. The features array names the media feature and supplies the value dark. Replace the example URL with the page under test.
If the page is already open
When a test changes the emulation after navigation, reload the document before capturing it:
driver.execute_cdp_cmd(
"Emulation.setEmulatedMedia",
{"features": [{"name": "prefers-color-scheme", "value": "dark"}]},
)
driver.refresh()
driver.save_screenshot("dark-mode-after-refresh.png")Reloading gives the page a chance to reevaluate its media queries and theme initialization code.
Wait for the dark UI before capturing
A screenshot taken immediately after navigation can catch a loading shell, an animation, or a theme switch that has not finished. Wait for a stable, page-specific condition instead of using a large fixed sleep whenever possible.
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Wait for a theme indicator
from selenium.webdriver.common.by import By
from selenium.webdriver.support.ui import WebDriverWait
# Example: the application adds this class when dark styling is active.
WebDriverWait(driver, 15).until(
lambda d: "dark" in d.find_element(By.TAG_NAME, "body").get_attribute("class")
)
driver.save_screenshot("dark-mode.png")Change the condition to match the application’s real markup. A site may use a data attribute, a class on the document element, or only CSS variables.
Use a short delay only when necessary
import time
time.sleep(1)
driver.save_screenshot("dark-mode.png")A delay is simple but less reliable than waiting for a selector or state. Keep it as a fallback for pages whose readiness cannot be expressed through the DOM.
Verify that the screenshot is actually dark
A successful screenshot call proves that Selenium wrote an image; it does not prove that the intended theme was applied. Add an assertion that is meaningful for your site.
Inspect a computed style
background = driver.execute_script(
"return getComputedStyle(document.body).backgroundColor;"
)
print("body background:", background)
assert background not in ("rgb(255, 255, 255)", "rgba(0, 0, 0, 0)")
Choose a property whose expected dark value is stable in your application. Exact colors differ between designs, so do not copy the example assertion without checking your page.
Check a theme marker
theme = driver.execute_script(
"return document.documentElement.getAttribute('data-theme');"
)
assert theme == "dark", f"Unexpected theme: {theme!r}"
If the site exposes no marker, compare a known region or inspect a computed CSS custom property. Keep this verification separate from the screenshot assertion so a light capture cannot silently become a passing fixture.
Capture a single component instead of the whole page
Selenium also provides element screenshot APIs. Locate the component after the page has switched themes, then save only that element:
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from selenium.webdriver.common.by import By
card = driver.find_element(By.CSS_SELECTOR, "main .pricing-card")
card.screenshot("dark-card.png")Element capture is useful for a component regression test or a small visual artifact. Use driver.save_screenshot() when the browser viewport, including surrounding layout, is what you need to inspect.
Site dark theme versus automatic browser darkening
Emulated prefers-color-scheme
The Python example tests the site’s supported dark mode. A page can respond with CSS such as @media (prefers-color-scheme: dark), JavaScript theme selection, or both. This is generally the right condition for a dark-theme visual regression test because it exercises the design the site intended to provide.
Automatic dark rendering
Chrome also has an automatic dark-rendering mode that transforms web content even when the site has not implemented a dark theme. That is a different test condition from advertising a dark preference to the page. Use the CDP automatic-dark override only when your requirement is to test that browser transformation; do not substitute it for a site-theme test. The command and parameters can vary with the Chrome/CDP version, so verify the current protocol support before building a long-lived test around it.
Full-page, viewport, and layout considerations
A normal page screenshot represents the current browser viewport. Set the window size before navigation when a breakpoint matters:
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driver.get("https://example.com")
driver.save_screenshot("desktop-dark.png")Run another capture at a mobile-sized viewport if responsive dark styling is part of the requirement. Keep viewport dimensions, device-pixel settings, fonts, and browser version consistent across visual-regression runs; otherwise a color change can be obscured by layout or rendering differences.
For a long document, decide whether the test needs the viewport, a particular element, or a browser-specific full-page facility. Do not assume that a viewport screenshot includes content below the fold. If you use a full-page technique supplied by your Selenium/browser combination, test it separately because support and behavior are browser-dependent.
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Troubleshooting dark Selenium screenshots
The image is still light
- Confirm that the CDP command ran before
get(), or callrefresh()after changing it. - Check that the page actually implements
prefers-color-scheme. A manual theme toggle may use application state instead of the media feature. - Verify a marker or computed style before saving; the screenshot method itself does not turn on dark mode.
- Look for an iframe. The visible content may come from a separately rendered frame whose theme logic differs from the top document.
unknown command or an invalid-parameter error
CDP commands are tied to Chromium’s protocol implementation. Update or align Chrome, the driver, and Selenium, then check the command spelling and the features object. Avoid silently falling back to a light screenshot when the emulation call fails; fail the test and report the browser versions.
The screenshot is blank or shows a loading page
- Wait for a page-specific selector or theme state.
- Check network failures and JavaScript errors in the test environment.
- For headless runs, use a realistic window size and ensure required fonts and assets are available.
- Capture after the application’s route transition, not merely after the initial document load.
The screenshot differs between machines
Normalize browser version, viewport, device scale, installed fonts, locale, timezone, and animations. Disable or wait for transitions where your test permits it. A color-scheme preference does not eliminate other rendering variables.
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Wait until the element exists and is displayed, scroll it into view if needed, and make sure the selector identifies one intended component. A stale element reference means the framework replaced the node after you located it; find it again after the update.
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Keep the CDP boundary small
Wrap the emulation call in one helper and record the browser and Selenium versions in test logs. This makes a protocol change easier to diagnose and limits the number of tests coupled directly to CDP.
Prefer deterministic readiness checks
Wait for the application’s dark-state marker, fonts, and data required for the screenshot. Fixed sleeps can pass on a fast workstation and fail on a busy CI runner.
Store useful failure artifacts
When an assertion fails, retain the screenshot, the current URL, and the detected theme marker. Those artifacts distinguish a failed color switch from a navigation, selector, or layout problem.
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Browser compatibility and WebDriver BiDi
The documented route here is Chrome/Chromium CDP. CDP is not a stable, cross-browser testing API, and Selenium describes its access as temporary while WebDriver BiDi becomes available. BiDi is the W3C-standard direction for cross-browser automation, but an equivalent, universally available media-emulation command is not established by the documentation covered here. Treat CDP as the known Chrome solution, and verify current BiDi support before planning a migration.
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Frequently Asked Questions
Can I use the same Selenium script to test a manual dark-mode toggle?
Yes, but that is a separate condition: perform the application’s documented toggle action, wait for its state marker, and then capture. The CDP preference test covers only behavior driven by the page’s color-scheme preference.
Does changing the preference persist for later browser sessions?
No. Apply the emulation in each new WebDriver session before the navigation that needs it.
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