i dont think this will ever happen. look at my posts here. I've been asking for this for 10 years minimum, and let me tell you, not a SINGLE tweak or improvement during that time.
I posted recently (a few weeks ago) after having upgraded my subscription...100% everything exactly the same as it was before. I am starting to question the amount of development actually happening in MLO, it must be super minimal.
I just spent a full week, many hours, playing with the print template. Whoever says this thing can be customized is completely misinformed. Yes, there is css...yes it can be modified like css normally can. However, it will make NO DIFFERENCE. Trust me, I've been trying for 10 years! go ahead, and tweak it....let me know when you get a result you are satisfied with. And post it!
What needs to happen is a total overhaul of all the graphical presentation elements. On screen, printing, and app...all synced. The customizing of the on screen colors has major hindrances also, to the point where i'd recommend that users dont even bother tweaking them. it will be an exercise in extreme frustration, just like the printing. minor changes will be ok, but dont try to create your own fully customized theme, you will never get it the way you want. i have posts on this. the color choices, too...like what is that? not even rgb or normal hex colors. there is no way to figure out what colors you want and copy/paste etc., in addition to other problems.
anyway...i hate writing like this for a really good program. but man, youd think after 10 years there would be some improvements.