Since there is not official dark theme for opus and there are so many created by huge community around dopus it is hard to choose one 
I dont know if theme could couse any slow down or anyting like this or change behavior. So I am little bit careful when going through themes and choosing one I should try.
Then I asked myself if DOPUS developers (Jon, Leo...) like the light theme which dopus has as default or they have some themes from the community they like and are using?
For actual themes (.dlt files), they just change the color settings, and cannot slow things down.
(Well, unless there is a bug involving combinations of cosmetic settings, but we don't know of any in 12.17.)
Some "theme" posts include complete configuration backups (.ocb files) which can affect pretty much everything, as well as trashing all your preferences. We don't recommend using those, to be honest, although I doubt many of them would affect performance.
There will be an official / built-in dark theme eventually, but we want to overhaul the themes system a bit before that happens.
Thank you Leo very much. Having official dark theme in dopus will be wonderfull. In meatime I will try to play with the colors myself or try some DLT files to change them.