One of the nice way you can customize REAPER is by changing it’s UI. I have already spoke about various tools to do this.
But if you want to go further, you can also create your own. You can start from scratch or start from an existing theme (making a “mod”), if you just want to modify some element of an existing one.
Looking for REAPER theme is a fun thing to do for beginners. When I started REAPER few years ago, I found Lerian’s Default Analog, which I like for its warmer tone (it was based on REAPER v4 theme, but orange dominance instead of green). I started to mod it, shared the result publicly, and little by little I continue to add tons of features like horizontal panning in mixer, some shadows, extra buttons images, change the responsive behavior etc… Except the main buttons, it is now way different than the original!
Years later, and the despite the nice evolutions of the native theme, I still use mine and continue to maintain it if I found an issue. Most recent updates include having FX in right part of the TCP, which is extremely convenient (see screenshot below), and also a dark theme, which is the one I use now (see screenshot above).
It surely not the fanciest theme out there (no special support for HDPI for eg) but it is comfortable and kinda neutral which makes it easily readable. It has more than 15 000 downloads, and I sometimes saw it on YouTube videos, which is kinda cool!
You can read more about all its features on the dedicated article on my website, and download it for free from there if you want to try.