Improve themed color definitions
The current color definitions are a bit messy since they are partially redundant and used inconsistently. It would be good to switch to 16 base colors that are mapped to ratatui’s 16 default colors and a set of custom grayscales.
The new names could potentially be indexed, e.g. base_01, gray_01 instead of cyan etc. since the user’s color profile might not reflect that color anyways.