@Fredeke in my short time with Reaper I found that there are many, many options and at times it can be overwhelming.
I started simply and gradually went deeper into customising Reaper.
Backup your current configuration
You may already know this, but before anything else I would backup your current configuration:
1) Select the Option menu
2) Select Preferences
3) From the Preferences dialog box:
4) Click on the first option on the left: General
5) On the panel on the right click on Export Configuration - a list of all the configuration items will appear after a few seconds
6) Click on Save
7) A file selection box will appear - enter a filename for your backup e.g "My Config Backup 01"
8) Click on Save and your current config will be saved so you can always revert back to it. For extra safety I tend to save mine in my user document folders.
Install a simple theme
I would start with a simple theme to get a feel for it.
My system is Windows 10, so I don't know if the following steps work the same way on MacOS:
1) Go to:
https://stash.reaper.fm/tag/Themes
2) Select a theme you like the look of e.g
https://stash.reaper.fm/theme/2293/JanneVThemes.zip
3) Find the file you downloaded, and unzip it
4) inside the unzipped folder you'll find two files:
JanneV.ReaperThemeZip
JanneV2.ReaperThemeZip
4) Next open Reaper
5) Drag the one of the files above onto the Reaper window
6) Reaper will automatically install the file for you and change theme
Switching Themes
To switch between installed themes:
1) Select the Options menu
2) Select the Themes entry
3) You should see a list of installed themes
4) Select one
5) That's it
Where does Reaper store its Theme files?
Reaper makes a copy of any new theme it install automatically.
To find the location on your machine:
1) Select the Options menu
2) Select the Show REAPER resource path in explorer/finder...
3) Explorer / Finder will open that location
4) There should be a folder called ColorThemes - that's where the theme files are stored
5) You can install themes manually by copying them to that folder directly and using the Switching Themes steps above to select them
I hope that helps as a start. Let us know how you get on