[WIP] Use php-debugbar dark theme, move to variables

[barryvdh]

This is pending on php-debugbar/php-debugbar#703

• Move hardcoded colors to CSS variables
• Add a dark theme for php-debugbar, based on CSS variables
• Move hardcoded colors for laravel-debugbar to CSS variable overrides
• Tweak Laravel-debugbar darkmode to only use CSS variables.

This should make it easier to maintain both themes, if the existing variables are used. And a lot less CSS duplication.

Some color overrides should probably still be done in php-debugbar instead of here.

Note: This requires the feat-darktheme branch for debugbar to work.

[barryvdh]

I think this is done, but requires the latest php-debugbar beta.