Tabbed Application Mode: Provide tab colour customisation

[lukewarlow]

Currently the tab_strip field doesn't allow any customisation of tab colours, and Chrome's current experimental implementation uses the body's background colour for the selected tab and the app manifests theme colour unselected tabs.

I can think of use cases where a developer might want to customise this to improve the UX.

Example 1:

The developers favicon supports dark mode browser UI, using the link media attribute, but their site doesn't support dark mode yet. Currently the white background colour would be combined with the dark mode favicon leading to no or low contrast on the selected tab.

Example 2:

The applications background colour could be the same as their theme colour which would make it hard to distinguish between the current tab and other tabs.

Ideally if a way to customise this was provided it'd support at least a choice based on color-scheme media queries but should probably also account for contrast ones.

[alancutter]

Re example 1: I think any half implementation of dark mode will have problems like this, the site should hold off until they have a full dark mode implementation. I would not optimise for this scenario.

Re example 2: Setting the theme colour and background colour is a valid configuration, if the ability to see the tab is hard that's on the user agent for not providing enough contrast rather than on the dev.
For reference this is what it looks like in Chrome OS's experimental implementation today: