Change Button Colors

I would like to change the sage CRM button background colors to a different color. So far, I have tried to change background color styling in the a.er_buttonItem section in the ergonomic.css file but the changes did not reflect as expected. I ended up changing the background color in custom content which seems to do the trick but the problem with that is I would have to through all buttons in all screens. Anyone with a better approach to this? I am using sage CRM 2018 R3 the buttons in question have been attached.

Kind regards

Kind regards