2shine is built to give you many different customization possibilites. You will have to prepare for your customization by installing node.js and some dependencies that are needed to convert your changes to CSS and JS.
You should now be able to make changes to your own 2shine Theme.
You can find all of the different stylesheets in the src/scss folder.
In this folder you will also find some files that have the prefix dnn those files are used to make Dnn specific style changes like changes to the Dnn UI.
You can find the standart scss variables in the 2shine skin folder. From there you go to src > scss > _variables.scss. There are variables with the prefix "tosic"which just means those are variables that we prepared and are set for our preference. Most of the variables are pretty self-explanatory but we will take a short look at the important ones anyway.
To learn more about the variable go to the _variables.scss file and take a look at the comments there or go to take a look at the Sass Bootstrap5 documentation.