Since 2.23v back office is bootstrap 3 powered, the easiest way to change back office theme is
If you want to change front office theme you can, use style widget's using internal themes or if you want to change front office widgets content you can do the same steps just for design/customtheme/vendor/bootstrap/css this folder. Also you can change themes using extension.
If you have created some cool theme share it with us!
Developing application takes a lot of time. You can support application by donating. There is no company behind this application and it takes away my free time. Every donation matters and does not matter how small it is!