I'm not sure if this has been suggested yet and I don't care to search through all 10 pages of this to find out.
This is simply what I did to fix my Security Token issues for my custom theme for my board.
Go to your Administrator Control Panel, then choose:
Styles & Templates >> Search in Templates
Inside there you will use the "Find and Replace in Templates" function.
Where it says "Search in Style" you will choose the custom style that is giving you problems.
Where it says "Search for Text" put:
Code:
<input type="hidden" name="s" value="$session[sessionhash]" />
and where it says "Replace with Text" put:
Code:
<input type="hidden" name="securitytoken" value="$bbuserinfo[securitytoken]" />
<input type="hidden" name="s" value="$session[sessionhash]" />
Then choose "No" for the "Test Replacement Only" option.
And finally choose "Yes" for the "Case-Insensitive" option. (Just to be sure. =D)
Click Find and then keep hitting next till it updates the skins.
That's what I did and now it works like a charm. =D