Hi @Flamestar,
try running the following terminal command first (Ctrl + Al t+ t):
sudo apt-get install mate-desktop-environment-extras mate-core && sudo apt-get remove ubuntu-mate-themes && sudo apt-get install ubuntu-mate-themes
Then try logging out and back in or restarting!.
If that fails, try the following guide and do as @stevecook172001 suggests, a fresh install BUT DON'T FORMAT YOUR HOME!: