An easy way to customize the backend is to add your custom css to the end of the active theme.
Location: /usr/local/hestia/web/css/themes - .min.css
Open the active theme’s file and right at the end add your custom css values.
In my case all I wanted is to give the .top-bar a different color to help me distinguish which server I’m on since I have several installations.
Here we’re benefiting from CSS cascading effect. No need to create a whole new theme since it’s a minor modification.
Maybe this is a good request for future update, to include a color picker or field to change the top bar color to help us choose a color for each server. Not a big thing but it helps when you have many control panels open at the same time.