Contact plugin developer to resolve your issue as this is third party plugin, we do not provide assistance on 3rd party plugins. 

I already contacted him, but he said he doesn't know how to help me

Send a complaint to moderation team moderation_team@oxwall.org

dealing with aron is painfull

his plugins are a mess to try and fix or do anything with 
one huge php file that does everything and try to change or fix is hard to do 

but what you want to do if you use inspect element and fine the css that drives the colors 

then search the file dont even try looking as  his plugins have css form all is pulgins in one huge mess 

on finding it you can change the file or add the css in admin theme edit 

I tried to modify the css stylesheet but it didn't work. The admin panel says is not in the base css, because the colors are in a file of the plugin.