Is it possible to modify or add themes?

Is it possible to modify or add themes?
The theme only gives a light or dark option. I would like to change the background color of the theme. Is it possible to change this color or add a customized theme? If yes, how do I do it?
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You cannot add themes. The extent of your ability to modify a theme is limited to the UX >> Brand tab.

following for this feature…i want to have a “sepia” theme, with yellow in background and black on the text for best reading

I really appreciate it that you can give such instruction in detail. Thank you kindly.

Maybe the group can add this custom function, though it may be a challenge.

I did.