buttons css - where to consistently insert css

Tammie,
getting a little bit confused with the buttons-background on every update:
They turn to the original "grey" each time..
I am trying to live the message of this wpmu-featured article that color matters.
(https://premium.wpmudev.org/blog/how-to-turn-your-website-into-an-automated-sales-machine-part-iv/)

I changed the css and tried about everything:
In custom.css the buttons are not overridden.
in styles/dark-orange it worked for sometime, but ot for last update-also not for ie.
in core: plugin it still seem s to work, but is always overwritten with every update ofcourse...

What is a clean solution to have customized buttons (at least colors)?
Any solution ahaed?