As for customizing fonts and their sizes, you can do that in the tweaker, just make sure you have all the windows open you can...mixer, timeline, docker, etc., and before changing one, make a note of its original setting. Then change one to something wild like dingbats or webdings, click apply, and you will see which element was effected.
This is a big help because themers may use any of the numbered WALTER fonts for whatever they want, the line that says 'vol/pan font' or 'track label font' may not be those things, in fact. There are also fonts you can change like 'timeline font', etc. that are not listed with the other WALTER fonts, they are in other places in the tweaker window. So being aware of those and your method for ID'ing fonts, you can now go about settings every font to your hearts desire. EXCEPT you cannot set the color for some reason, hmmm, wow REAPER, well geez, you mean 5 different tweaking scripts, 5 different locations for dev info, and many levels of changing and overriding setting and....give me a break...yes, you actually have to unpack the theme, edit the 'rtconfig' file and all of the font settings inside of it for each and every layout of the theme you use, and that is, if you can decipher anything at all that is going on because the newer default themes are not really WALTER at all they are all math and variables and conditions and macros and yea, good luck.
I suppose you could just write a macro changing the color of every font in the theme and place it at the end of the file. Anyhow, if you want to make a high contrast version of a default theme or any theme, someone here may help you. Someone may take a swing at it for you. More info would be needed and such...
Part 2:
There is a couple of actions that are used to help themers I am away from REAPER atm but I think they are called something like theme helper or element finder if you search actions for the word theme and the word element you should see them. Play with em and see how they may benefit you. Also you can see some names of items by unpacking and opening a theme's resource folder. Further, there is a list of images, a list of cursors with in action examples, and more available as sdk materials.
https://www.reaper.fm/sdk/walter/
https://www.reaper.fm/sdk/cursors/cursors.php
As far as anything not covered in any of this, which may be foreign to some because they are descriptions of OS elements or waveform elements or general DAW elements etc., and things which vary from DAW to DAW like 'playhead' and 'play cursor'...many different conventions are used in the tweaker menu that are kind of out of context, and you are not the first to have difficulty. Bringing me to my point; whatever items you cannot glean from any of the above resources, just post again the exact desriptions item by item and someone will explain...like 'main window 3d highlight' or 'window text' (these are actually overriding an OS elements).