Allow env level configuration of default site colors for both light and dark modes

We would like to be able to setup values controlling the site's display to allow different branding of the site, at the level of env file, without the need for anyone hosting a copy of the code to manipulate the core files.

This would involve having env var that would set overriding values for CSS values such as:

--background-start-rgb, --background-end-rgb, --color-text, --color-background, --color-row-even, --color-row-odd, and --color-row-hover