Could you please make an option to setup background? As you can see it is useing global transperency and it is quite unreadable.

Transparency is handled by ElvUI directly.
YES but it is GLOBAL function and it changes also chat frames and other parts of ElvUI.

I would have to look into the API more since this addon just uses WOW's EasyMenu system for this. It may only be possible to set the alpha (transparency) globally.