Before installing any themes, it is recommended you first copy the original theme to a folder named GH3Default in the THEMES folder, so it can be restored after applying other themes.
Its main files are global.pak, global.pab, and global_sfx.pak.

For Full Themes: Extract the files (global PAK/PAB) from the ZIP to a new folder in the THEMES folder named after the theme, and then drag the folder onto __changePAK.bat to apply it. Alternatively, it can just be copied to DATA/ZONES.

For Partial Themes: Extract the file(s) (.SCN/.TEX/.GFX) from the ZIP to the THEMES folder or copy the ZIP to there, and then drag the TEX or ZIP onto __changeTEX.bat to apply it.