I have recently upgraded DNN version 7.0 to version 9.1 (via extracting package into DNN folder and executing /Install/Install.aspx?mode=upgrade)
All steps finished with Success result, but I was not able to build a web site. I had to delete following files/folders manually:

• /Admin/Containers/admin.ascx
• /Admin/Containers/solpartactions.ascx
• /Admin/Containers/solpartactions.xml
• /DesktopModules/DevicePreviewManagement
• /DesktopModules/MobileManagement
• /DesktopModules/Admin/Taxonomy
• DesktopModules/Admin/MobilePreview 
• /DesktopModules/Admin/FileManager

After those deletions I was able to build a web site and deploy 

So now I have several other issues with DNN

• If I click on Search tab in site settings, I receive internall server error when page is calling  /API/PersonaBar/SiteSettings/GetBasicSearchSettings page 

I would like to modify existing template just for one page, so I copied existing template, renamed the folder and choosed it as a default template. I have noticed that site is using default.css stylesheet from directory /Resources/Shared/stylesheets/dnndefault/7.0.0/ How can I change it to /Resources/Shared/stylesheets/dnndefault/8.0.0/ ?

How can I change layout just for one page, but not for  the whole website?

Thank you,

Rudolf.