Safari overlays its own nav at the bottom 10% or so of the screen. This
makes buttons in that area virtually unclickable, so to ensure buttons
there are reachable, we need to add enough bottom padding to menu panels.
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.
