[CHORE] Consolidate CSS colors into a single set of variables

[ryankwirth]

Purpose

Currently, our color codes are scattered throughout component .css files, and there are quite a few identical ones in different components.

💡 Proposed Implementation:

Create a single file consisting of light- and dark-mode colors defined as CSS variables. These should use the naming scheme from the Material Design color system, like colorPrimary, colorSecondary, colorSurface, etc.

This will allow future components to use a consistent set of colors and improve maintainability if a color needs to be tweaked.