There are currently 4 different themes on the client that can be changed from Settings > Themes. You can also set a custom picture as a background. I'm afraid that you cannot select the colors yourself. However you can post this to the forum's Ideas section (https://www.3cx.com/community/forums/3cx-ideas.57/), for the appropriate department to consider.
thanks, but the problem is - this settings are independent from my account. I can select a theme and a background - and this will be fixed until I change it for myself.