I was suffering from white Rambox panel for some time, I even thought to change its sources and build it manually to make it dark. But finally I found a solution, even though it’s temporary - you’ll have to do it every time you run Rambox.

Basically you need to open the Developer Tools window of Rambox (not the Service) by clicking on Window -> Toggle Developers Tools, open Console tab and execute the following piece of JS:

document.querySelector('.x-tab-bar').style.backgroundColor='#20272D';
document.querySelectorAll('.x-tab').forEach(function(el){el.style.backgroundColor='#444';});

First line sets the color of the bar and second of the tabs. Result will look following way: