Themes are almost without exception stored in zip format. This means you'll need a 32 bit version of WinZip to extract themes. To install themes, you'll need to use some theme application. But there are several other programs that work just as well, e. g. Desktop Themes or XTheme Manager .
Most themes are very easy to install.
Just unzip them to your theme directory and install. But some themes are improperly coded. The most common mistake made is that the .theme file refers to directories or files that are missing or have nothing to do with the theme in question. Luckily, such mistakes are very easy to fix. A .theme file simply tells windows where to find wallpaper, cursors, icons, etc.