My advice is to search the Internet for some good resources on how to learn the Bash programming language, or else the "POSIX shell" (which is very slightly different from Bash). This is the command line language used by most Linux distros by default. POSIX shell is a complete programming language with built-in data structures, control flow like conditionals (if statements) and loop syntax such as for and while, and function composition by way of shell pipes. Combine these with tools like find and grep and you can accomplish quite a lot with just a single line of code.

Also, I highly recommend you practice using Vi or Vim as a text editor until you get somewhat fluent. It isn't strictly required, but I recommend it simply because most people who use Linux as their daily driver also use these tools, and it helps a lot when communicating with other people in the Linux community, especially when it comes to solving problems. You want some fluency in using apps that the majority of people in your community are using. Also it is a good place to practice writing shell scripts.

Also not required, but learn a bit about Emacs as well. Learn how to use Dired (an Emacs app for working with ls output interactively), learn how to open an edit a file from Dired, learn how to run find and grep from within Emacs. Learn how to run shell commands in Emacs and capture their log output. Also learn a bit about how to use Org-mode. (Shameless plug: I have my own series of blog posts on how to do these things.) This handful of basic skills will get you a very, very long way.

I can't comment much on customizing Cinnamon, but Cinnamon uses the Gtk toolkit, and so you can use any of the good Gtk themes out there to customize Cinnamon. Check out the UnixPorn community for more in-depth advice on that topic.