Configurability
How tough is it to tweak?
Together with all the extensions, there are a huge number of themes for all the contenders featured here in this
Roundup. In Gnome, you choose themes for different categories; applications, cursor, icons, shell and sound. They come in separate files, and they use CSS for the components. You also have many SVG files to represent the animations. This makes it easy to install existing themes after you have downloaded them. You will not be able to search from the install tool, while in KDE you can search the official site for these things straight from your tool.
In Gnome you can add new extensions using the extensions webpage. It has a local section that shows your currently installed extensions. If you put in too many or you suspect that one is crashing your system, you can turn off all of them and start from the beginning without removing them.
In KDE there is a special application just for adding new themes and desktop elements. This is a little better, but other solutions exist to save all of it as a single file. The repositories are huge, so you will spend most of your time looking for your own personal style. The files themselves are easy enough to understand if you have made any webpages, as they are either made with CSS or Javascript.
Awesome uses the Lua language, which most people should be able to figure out just by reading the files. This way of changing the look is a bit more cumbersome, at least for non-programmers. The idea, of course, is that developers and others who use the command line a lot are the primary users. As a consequence, the configuration files are scripts. In Awesome, Lua scripts call other scripts or binaries. When you get used to it this is actually faster, but regular users will face a fairly steep learning curve to overcome initially.
To configure Regolith, you copy the default i3 configuration file from /etc/regolith/i3/config to your home directory ~/.config/ regolith/i3/config. The file is a straight text file where you set parameters for i3.