A common problem asked in the forum is:

‘Why does the Forum appear without a skin I just uploaded?’

or

‘Why is the new skin I just uploaded not appear in the menu selection?’

Skins generally come in archives that decompress  into a folder with the skin name.

Although smart webmasters would instantly believe that this folder properly belongs in the wp-content/plugins/simple-forum/skins/ directory, this may in fact be wrong.

Many Skin archives have ICONS as well as SKINS

Open the archive directory and check if there are two folders inside.

If there are, chances are that one is named icons, and one, skins, and inside these two directories there will most likely be two more folders, each with the same name as the skin.

It is these internal skin named folders that need to be uploaded and placed in wp-content/plugins/simple-forum/icons/ and wp-content/plugins/simple-forum/skins/, respectively.

This should solve your problem.