Ability to specify custom background colors and text colors in Rill

[AndrewRTsao]

As an extension of our custom themes support, customer may be interested in setting custom background colors and/or text color customizations to further personalize the Rill dashboard to align with their internal UX themes / needs.

• For custom background color, being able to set custom background colors (especially for the Cloud and Dev sections) would allow better integration of the Rill dashboard into existing themes.
• Being able to change / modify the text colors would allow the customer to further tailor the dashboard to their preferences, ensuring a more cohesive and visually pleasing user experience.

Context: https://rilldata.slack.com/archives/C05JLJ7E405/p1704773399988359?thread_ts=1704721037.316469&cid=C05JLJ7E405