Dom, I think that a lot will depend upon what your moodle partner is willing to do, and it may also depend upon which theme you are using right now. (I don't use a partner.)
When I was using moodle's MORE theme, I did customize MORE with custom CSS, but this was done in a dialog box within moodle and this MORE theme. The beauty of this approach was that when I upgraded my moodle, the custom CSS persisted. Trying to customize a theme by writing custom php code could involve a few more maintenance issues when upgrading, which could bother a partner. But I would think that simply suggestion some custom CSS be put into MORE's dialog box could work, because there is little maintenance overhead.
Now I use BOOST, and I have used some custom CSS in BOOST's dialog box, and a supporting present.scss file. This involves two things. The more "things" that get added, probably the less that your partner will be willing to tolerate.
(Here is a link to my webpage that provides some resources to a presentation that I gave at MoodleMoot US.)