ok, we started from the amazing theme xswatch from Richard !
Now, all devs and designers work on xswatch4 (Bootstrap 4).
Have a look to the readme file to understand that you can control color style.
Very important but very easy (thank you Richard)
You can change style via this site : https://bootswatch.com/
For https://www.monxoops.fr/, we used this style : slate
For sure, we had lot of special css but the start was on wswatch then => slate v4.
Many devs give bt4 module templates for xswatch4 (overloaded template).
Humm last words :
we used XOOPS 2.5.11