What resolution you do use and what theme?
I guess you mean theme which gfx elements must be rescaled to be displayed with expected size. The rescaling causes image quality loss
in fact most of all the 3rd party themes that I tried
Most 3rd party times on Stash were built for 1920x1080 and lower resolutions since they were made for pre-v6.0 Reaper.
Your system is likely scaling the images, giving you lower quality images, as MaXym suggested.
how come that certain themes are so blurry ?
Linear Mode
