I would not spend time on this skin TBH.
It's old, xhtml not html5, not responsive and the question is if you will get the menu to work in older browsers.
I think using a newser skin and renaming the panes to the pane names you are using is less work.

There are two things you need to do if you really want to get it to work in DNN 8+.

A. Replace the Solpart menu in the skin with DRR, there's a drop in replacement for it although there will be some styling issues AFAIK.
B. Remove all references to Solpart from the containers.

HTH

I red that post, but it didn't help at all. Here I found something related to SolPartMenu, but again I need to repeat that I didn't found any reference to SolPartMenu inside index full.ascx of MinimalExtropy skin.

Did I focus to right file ? Or is my issue really related to SolPartMenu ?

Here I uploaded what I mean by "menu and submenus are all expanded".

I didn't read your initial message carefully enough.  Those refer to containers, not the skin.
 

The easiest fix is to make your default (and other) container be one from Xcillion.  If you need the "blue" part, you should be able to take a closely matching Xcillion skin ad add appropriate styling.  I'd copy and rename the container and style it.

Ok, as I created this Theme about 10 years ago, here's an update for DNN9.

https://github.com/Timo-D...eleases/tag/01.01.00

Great, thanks. Apparently it can be applicable also to DNN8.
There remain only issue where icons of buttons in Edit Mode are not displayed (pen, wheel, 4wayArrow). Here is screenshot. How to fix it ?

BTW: Issue with "menus and submenus are all expanded" was solved by clicking by Admin > Themes > Parse Theme Package.
Not sure what it does, but it helped :-)

There remain only issue where icons of buttons