This is way how I will use custom css in new or updated themes for e107 2.3.X
Custom css is inline css inserted directly in some of my themes via Theme preferences. Normally, it uses the standard text area field.
This article is an explanation if you find this is in your theme because it is not a standard solution. And it is an overview for me for implementation in the next theme.
The code inside head tags
Custom theme shortcodes