(Note to self: I need to figure out a way to keep these references handy.)
You may wish to review my posts in another forum (now archived) as well as the conversations that those post were part of, to get some good advice and hints on how to approach this new and rewarding adventure that you have undertaken. Take it easy and don’t beat yourself up if things don’t go as planned at first attempt. Failed attempts are just lessons on how NOT to do things! 
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Absolute Beginner - #7 by ericmarceau - Support & Help Requests - Ubuntu MATE Community
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Ubuntu MATE bugs - #10 by jaymo_chicago - Ubuntu MATE Community
I hope you find those helpful.
For any questions, feel free to ask the Community! You will find them more than eager to provide real help.
If you have a bit of programming experience, you may even wish to delve into customizing some aspect of your theme, possibly compiling the component directly from distribution source. In that case, this posting introduces you to what I did for a very limited customizing of a theme on UbuntuMATE. My purpose in attempting that was to deal with the “limited size of the edge-detection zone” at window edges for mouse “grabbing”.
I tried to be as explicit and explanatory as possible, to the point of embedding a “workflow” approach in the provided script. That might be helpful to get your head wrapped around what some of the Distros and Packages expect you to do when trying to custom-build.
Good luck with everything!