You can customize OpenPanel UI colors and fonts using custom CSS code. Here is an example 'I feel blue':
Source: https://gist.githubusercontent.com/stefanpejcic/26bc386ba4ace00a29ff0c63e33f5674/raw/dce487ffb100788d2be8fa1aaa54061edfa5bf46/custom_blue.css
Custom code is added on OpenAdmin > Settings > Custom Code page.
