Installing multiple copies of the same theme/component is possible, but you rarely need to actually have multiple copies installed. We've seen many times new admins installing duplicates of components because they were unaware it was already installed. This PR makes the theme installer modal loop through the existing themes when you click on 'install', and if there is a theme with a URL that matches the URL you entered, a warning will show up and you will need to click 'install' again to proceed.
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.
