Draft: style: Improve contrast of privacy, screenshot indicators in light theme

To improve the contrast situation, I've added variables for foreground and background colors for the "destructive" color (used by the screenshot indicator) and added a new "privacy" color, also with foreground and background colors.

Having separate fg/bg colors allows appropriately contrasting colors to be chosen which fit both dark and light modes. For the "destructive" colors I'm using the colors from libadwaita: https://gnome.pages.gitlab.gnome.org/libadwaita/doc/1.4/named-colors.html#destructive-colors and for the "privacy" colors I've made color selections from the palette that achieve appropriate contrast levels.

Additional variables for dark-specific colors are made available so that the dark versions can be used on the "system" (transparent) panels used in the overlay, etc, which use fixed dark mode. This ensure that they do not incorrectly use the light-mode color variants.

The dark-specific colors are also used to fix the record button on the screenshot ui, which was incorrectly using the light version of $destructive_color even tho the panel is always dark, resulting in reduced contrast.

This is a more complete fix for #7112 (closed) that covers both the screencast recording and screenshare, camera sharing indicators.