I need to change the background colour on category , how can I change ?
thnx

https://docs.osclasspoint.com/customizations-of-theme-basic

Some themes also use category color set in theme settings.

thanks for the quick reply,

I tried it but I don't know how to set it

element.style {
    background-color: rgba(244, 67, 54, 0.15);

on Theme Customization
 thnx

Yes its set via style so that color is from category color, just 0.15 opacity only.
To override it manually, use !important
Selector is on your screenshot.

how can I put the background colour?

https://chatgpt.com/ can help a lot.
background:red;