Use theme colors for job overlay, improve dashboard on small screens

[yscik]

Changes Proposed in this Pull Request

• Expand the theme color detection script. Detect and calculate background and contrast colors based on the the theme.
  • Fixes color issues on dark themes, and daily chart colors when the link color is black.
• Improve job dashboard table, add column labels on mobile, improve breaking points, tweak some styles
• Fix outputting modal code multiple times

Testing Instructions

• Enable stats
• Install a few popular themes and check that the job dashboard and the stats overlay look reasonable out of the box
• Twenty Twenty-Three, Twenty Twenty-Four has a bunch of color styles that are easy to switch in the site editor, including dark colors.
• Check that the overlay also works in the admin

New or Updated Hooks and Templates

• job_manager_ui_theme_support_script - disable theme color support script

Screenshot / Video

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[gikaragia]

Although I think the solution is impressive, I also think that we may overcomplicate things while trying to make things look consistent. Maybe we should focus on simply providing the elements and let users handle more opinionated styles like colors?

Is it possible that having CSS variables makes it harder for our users to modify the colors as they will need to figure out how we use them? On the other side, maybe having variables is a common thing now and in the end they can always override everything so maybe it's not big of an issue.

[yscik]

So regarding CSS variables, the new styles rely pretty heavily on it, and it makes it pretty easy to customize things with just a few variables. This is an alternative to rewriting or overriding everything with CSS, which is what they currently have to do. I'm putting together a documentation page with some examples to get started.

This approach also makes it easy to add an UI on top in the future, where they can change the colors in the site editor or the setup wizard.

This PR adds the color detections so styles are not broken out of the box on various themes. Most of it is for the new overlay modal, where we'd end up with stuff like white links on white background if it's a dark theme.

[yscik]

Thanks for the review @gikaragia, all should be fixed now!

[gikaragia]

Looks great!