I found a smart way to override the backghround-color of the button.

Create an override for mod_cachecleaner/default.php

Line 19

Replace

With:

And put your custom color in "user.css" for the tag

.reg-cache-cleaner {}

I tried putting the class in the module, but it didn't show.

A tips for next version of Reg Cleaner, include unique class name also as default.