Now first find somethemes, these are not color themes under appearance but instead window manager themes that change the taskbar colors and such. In MX linux this is in MX tweaks and click at the bottom center the icon for window manager and the window that opens the tab you see is styles and the box themes it the big area at the left. Now these themes are not like appearance color themes that are in /usr/share/themes. These you don't need to sign in as root to install. YOu can go here "I got the page to make it simpler" https://hooktube.com/watch?v=BSNbUfJ6oTc . Now download the ones that catch your eye. Now go to your downloads folder and just select all and right click and choose extract here and a bunch of folders will be there. If you're in alphabetical sort change it to sort by date and all the folder will be at the top or bottom in Thunar. Now select the new folder and click cut. Now to to your home folder, at the top of Thunar click the view button and click on view hidden files. Now you'll scroll down and see a folder that is named .themes, just right click and paste into the folder. Now under window manager you'll see a bunch of new cool themes you just downloaded and just choose whatever you'd like for a new theme. I just looked it up and unlike MXLinux to change themes use the XFCE theme manager, should be on the whisker menu. Any problems I'll try to help, apparently in xubuntu the theme manager must be downloaded but that might have changed by now, I don't use ubuntu personally.