That editor is excellent :)
The only thing I wish there is bold #event - and maybe bigger... I want to see it faster. Sure some of the themes could be modified by the user but if you do that will be very good.

Also got a question: how do you restore original thumbnail of folder/file?

Another one: is it possible to put 2 stage icon - folder-opened folder?

Yet another post: is it possible to fold everything on the object? I must manually fold every event and {s.

Re: #event, currently it uses coloring akin to what GMS2 does. I don't think you can make the font bigger without causing oddities, but making the line or text brighter is fairly easy to do with a child theme. See how GMS2 theme works for inheritance and CSS in dark theme; CSS selectors are
Line number area: #main .ace_gutter-define:not(:first-child)
Code area: #main .gml .ace_line:not(:first-child) .ace_preproc.ace_define::after, #main .gml .ace_line:not(:first-child) .ace_preproc.ace_section::after, #main .gml .ace_line:not(:first-child) .ace_preproc.ace_event::after

Re: restoring icon, currently there's no UI for that - you'd open <project name>.css in project directory and remove the according line (they are fairly clearly defined).

Re: two-part icons, similar applies - would duplicate the line in CSS and change .dir to .dir.open.

Re: fold everything - yes, via Ctrl+M (keyboard shortcuts).