A first step to adding automatic dark mode color scheme switching. Adds a new SCSS file at `color_definitions.scss` that serves to output all SCSS color variables as CSS custom properties. And replaces all SCSS color variables with the new CSS custom properties throughout the stylesheets.
This is an alpha feature at this point, can only be enabled via console using the `default_dark_mode_color_scheme_id` site setting.
* Fixes position of the top arrow, and border width of both
* Merged top and transform properties
* .5px values are required to make arrow border appear the same with as overall popover border width
I did have a chance to try it out on a Windows machine with a non-high DPI display and there were no issues. And I just re-tested it with Firefox on macOS using the "Open in Low Resolution" option and it looked more or less ok (bearing in mind that it the whole app is then a blurry mess, and that using custom zoom levels causes the arrow to break slightly, but that was also the case on Firefox before this change)
