Hello,
Sorry to hear you are having issues. I can see Windowblinds icon on my taskbar. Refer screenshot below.

Moving forward with your issue. Need more information on your system.
- Windows full version and OS Build number. Please use Winver.exe.
- Is the icon don't show up as soon as you apply any WB skin? Or, it appear for a while but don't show up again after sometime, or not after a reboot etc.
- Do you have any other 3rd party application running in your system that change how default Windows 11 Explorer.exe behave? Such as Explorerpatcher, StartAllBack, Windhawk, OldnewExplorer etc
For your new question, to get back to default Windows theme. You just need to apply "Default theme" the first one listed in your Windowblinds 11 coniguration Windows : Style tab. If you found there are some colors that done change back to default Windows color, do this.
Open up Windows 11 personalization->Themes->Apply Windows theme. That should bring back all the color to default Windows theme.
Thank you.
Basj,
Stardock Community Assistant.