Allow a hook to alter the list of CSS files parsed by Color module

[docwilmot]

Description of the need

Color module provides preset color schemes. color.inc provides the schemes, the default colors which are swapped for scheme colors, but also a list of which CSS files are searched for these default colors. The CSS files in that list must also be ones declared in the theme's info file.

It would probably be useful if contrib modules which style their output, could also expose their output to color. So if a module created a new front page slider for example, it would be good if changing theme color schemes would change the slider colors as well, to match the rest of the display.

Proposed solution

A new hook, that lists CSS file locations to expose to Color module.

Alternatives that have been considered

None

Additional information

Note that this would (only) be helpful once we allow color info to be config, in https://github.com/backdrop/backdrop-issues/issues/2554

Draft of feature description for Press Release (1 paragraph at most)

Backdrop now includes the ability to expose module color styles to Color modules schemes.

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Advocate @docwilmot

[stpaultim]

I suppose that one needs to create a simple custom module to test this? Would it help if had a sample module to test with?

[jenlampton]

A new feature like this would also need tests to get into core (adding label).

I think @wesruv and @hosef are working on more modern alternatives to managing colors that might be suitable for 2.x. I'm not sure how much time/effort we should spend on the old color module if we decide to move forward with something different.

That said, this is not a big change, and if it benefits enough people it could be a nice improvement. If there are other people who would use this in 1.x please add a comment here :)

[hosef]

I think if people are working on fixes for the color module, we should still be getting those in. We are probably not going to have a working replacement for the Color module for quite a while.