Firefox addon that fixes low contrast text when using dark desktop theme
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README.md

Text Contrast for Dark Themes

Firefox extension that fixes low contrast text when using a dark desktop theme.

For users who don't want any native theming in web content, Firefox has a built-in solution. In about:config, verify that widget.content.allow-gtk-dark-theme is false. If you use a theme that does not have a light variant (e.g, Arc Dark; the light version, Arc, is a separate theme), then create a new string entry in about:config called widget.content.gtk-theme-override and set it to a light theme, e.g. Arc or Adwaita:light.

Otherwise, for users who wish to retain dark theming of un-styled input elements or entire pages, this extension tries to fix broken author css to ensure text remains read-able. Note that per the above paragraph, such users will likely want to set widget.content.allow-gtk-dark-theme to true.

Permission Requirements Explanations

<all_urls>

The extension inserts CSS and JavaScript into page content in order to fix colors, and requires URL permissions for all sites in order to do so.

storage

Extension settings use local storage.

tabs

Extension requires access to tab data to get active URL. In order for frames to see top-level tab URL, permissions for all tabs is needed, not just active tab.

webNavigation

Extension is triggered on web navigation events.

Input Elements

The primary purpose of this extension is to fix input elements, which are drawn with native widgets in the user's dark style. Many website authors assume that such elements are a dark-on-light style by default, and will only set one color in their styles. This results in poor contrast when drawn with the dark native widgets.

This extension checks if only one of the colors has been defined (foreground color, or background color/image), and defines the unset property as needed. Elements with both colors defined or both colors undefined remain unchanged. Elements which already have good contrast (e.g. author is running their own dark styling) are also unchanged.

In comparison to UserStyles.css or similar fixes, this extension tries to minimize changes that fix the contrast issue. Thus native styles are retained if possible.

Use with "Use System Colors" or custom colors

Some users with a dark desktop theme may use the "Use System Colors" preference to set the page default foreground/background to be light-on-dark. Alternately, users may manually set a light-on-dark style with their own colors. Similar to the situation with input elements, this can cause problems with author CSS.

This extension will detect the inverted default style and check for good contrast on the page itself. The check is performed recursively starting at the document root. Since colors are inherited by element children, the check bails early once it has found a explicitly styled element (whether due to author correctly setting both FG and BG, or extension making a fix). This keeps the performance impact of the page scan negligible.

Dynamic Elements

The extension performs required contrast checks (as above) for newly added or re-styled elements. Elements that have already been checked are not inspected, resulting in no slowdown with JavaScript heavy sites.

IFrames and SVG images

The sub-documents of IFrames and SVG images included via <embed> are traversed by the extension. The extension will try to correctly set the style of the inner sub-document based on the styling of the parent.

For users with light-on-dark default colors, any SVG images included inline within a document are also checked by the extension.

Note that SVG data included in an <img> tag cannot be accessed via JavaScript, and this extension is unable to fix such cases.

UI

The extension provides a toolbar button that allows users to either:

  • Toggle the effects of the extension
  • For users with light-on-dark default colors, force a custom stylesheet that emulates the black text / white background default. Both toggles apply to the current tab only, and are temporary. A page refresh undoes the effects.

Lastly, the extension options can also be accessed via the toolbar button menu.

Options

The extension provides some options for users to fine tune the operation.

Contrast Threshold

The extension does not modify element colors if it determines that they already have good contrast. The definition of good contrast is determined by this threshold. The extension uses the WCAG 2.0 definition of contrast (see 1.4.3: https://www.w3.org/TR/WCAG20/#visual-audio-contrast). Higher numbers make it more likely for the extension to change colors.

Delay

The extension can conflict with other extensions that modify page CSS. To allow the extension to run after any other custom CSS changes, a delay can be configured before the extension does any checking.

Override Lists

Some websites do not look good with this extension active. Two override lists are provided to avoid unwanted effects.

Disable List

The extension is disabled for sites in this list. If the site works well without the extension, but looks worse with it active, add the site to this list.

Force Standard List

The extension tries to force a default black text / white background style for any sites in this list. If sites don't looks good or end up too busy with the extension, this list can be used to revert to the stock browser style.

Known Issues

The following list of issues are known and cannot be fixed at this time.

AMO (addons.mozilla.org)

The extension does not function on addons.mozilla.org. This is a restriction enforced by Mozilla for any extension, presumably as a security feature. Unfortunately, the stylesheets on those pages are poorly written, and will result in black-on dark styling for many of the input elements.

Internal Pages

The extension cannot operate on internal pages, such as about: links, option pages for other extensions, and so on. As above, this is a built-in restriction of all web extensions.

Intentionally Hidden Text

Some site authors deliberately set a foreground or a background color with the intention of hiding text. When this extension encounters such an element, it has no way of knowing the poor contrast is intentional, and thus will fix the element. The extension should either be disabled for that specific site, or custom CSS fixing the offending element should be applied via UserStyles.css or similar.

SVG

SVG elements included as <img> cannot be traversed via JavaScript. If the SVG uses the 'currentColor' string for colors, these are pulled from the browser default colors, independent of the styling of the enclosing page. With no access to the SVG DOM, this extension cannot fix such cases. This is currently a major issue with with Wikipedia formulas, which will render in the user's selected foreground color, on a white background as set by the Wikipedia CSS. The only fix is to use custom CSS to force colors background on the parent page to ensure correct contrast (in the case of Wikipedia, force the background to be dark).

Element positioning

Some sites explicitly place elements outside of the normal layout flow. This extension relies on the fact that children inherit parent styles to keep fixes small and performant. The extension is unable to know that an element may have been moved above another element resulting in poor contrast. Forcing standard colors may alleviate the issue on such sites.

Cross-origin iFrames

When trying to fix all elements, and iframes are involved, the extensions must decide how to style an iframe based on what the parent does (if parent styles have been fixed, then iframe should be forced into standard colors, otherwise processed normally). This communication channel between the iframe and the parent is restricted when the frames are cross-origin, which blocks access to properties that the parent needs to determine if a fix is needed. This will result in the extensions failing to work in iframes on certain sites, which may require adding the site to one of the override lists.