Kevin Murek is the man behind the program which makes installing themes easy.
Link to .zip files (Windows): https://github.com/kmklr72/LMMS-Theme-Installer/downloads
Link to project site: https://github.com/kmklr72/LMMS-Theme-Installer
Step. 1
Download the latest file by clicking on it.
Step. 2
Find the downloaded file, right-click on it, choose the extract option.
Step. 3
Inside the folder, double click theme-installer.exe. Choose yes, even if the program does not have a signed publisher.
Step. 4
The program will open and automatically set your LMMS theme directory. Only change it if you want to save you themes on a specific location. Press next
Step. 5
Choose anyone of the theme listed and press the big install button. After a while the program reports that the theme has been successfully installed!