UberMenu’s skins are based on LESS files, which can be found in ubermenu/pro/assets/css/skins/.
To create your own skin, you would start with a similar skin’s LESS file, adjust the skin name to ‘none’ (to work with the disabled skin state), and adjust your LESS variables as you desire. It is recommended that you copy or redirect the output into a Custom Stylesheet (custom.css) Note the relative path resources which are required for the LESS build to work.
Using LESS is outside of the scope of the Knowledgebase, but there are many great tutorials on using LESS out there. If you choose this route, it is expected that you are already familiar with LESS.
Note that you can also create a custom skin based on the sample skin provided – see Custom Stylesheet (custom.css)