This patch changes the behavior for extensions with an embedded
options_ui page (open_in_tab not false), that have specified the
"dark" value in the "color-scheme" meta tag or CSS property, AND
with the user having indicated the preference for dark theme support.
There are no changes when any of these conditions have not been met.
The "color-scheme" CSS property is the standard way for extensions (web
pages in general) to opt in to automatic dark theme support, e.g. by
changing the foreground color to white. Prior this patch, the
background was unconditionally white, which resulted in unreadable text
when the dark theme is enabled. This patch changes the default
background color to "Canvas", which is a special keyword that is dark
theme-aware (almost black - `rgb(28, 27, 34)` by default).
When `browser_style:true` is used (which is the default in MV2),
extension.css is activated, which unconditionally specifies a black
foreground color. To avoid a poor contrast with the new theme-dependent
background color, the color is now white when the dark theme is enabled.
This patch includes comprehensive test coverage, but the only tests
whose behavior changed by this patch are:
- options_ui_dark (background changed)
- options_ui_browser_style_true_dark (color, background changed)
Differential Revision: https://phabricator.services.mozilla.com/D207540
8127bd5824