CSS roadmap proposal

[Michael Strauß]

Here is how I think we should be evolving JavaFX CSS over the next
period of time:

1. Media queries
Gist: https://gist.github.com/mstr2/cbb93bff03e073ec0c32aac317b22de7
PR: https://github.com/openjdk/jfx/pull/1655

This feature is already out for review.

2. System colors
Gist: https://gist.github.com/mstr2/afac42ab9587c1040a6cf7b091cee99f

System colors allow stylesheet authors to use colors that are defined
by the scene or the platform. Most notably, this includes the accent
color. We will also add new media queries to help stylesheet authors
deal with contrast modes.

3. With media queries and system colors in place, we can remove the
built-in high contrast stylesheets that come with Caspian and Modena.
Instead of hard-coding these special cases (and not even in a good
way), we can now use standard APIs to achieve a better result. This
will remove a big chunk of special-casing from the codebase and bring
us closer to an equal playing field of built-in vs. third-party
themes.

4. Dark Modena / Modena 2025?

5. Bold new theme?


What do you think? Please focus comments on step 1-3, and not on
bikeshedding a new theme.