Check the forum admin > wp integration > storage location panel for the location of your forum themes. By default they will be in /wp-content/sp-resources/forum-themes/

1. Locate the theme you wish to duplicate and copy the root folder to a new folder bearing the name of your new theme. So – if your  new theme is called ‘custom’ create that folder and copy the contents of. say ‘default’ into it.
2. Within your newly copied folder there will be a ‘styles’ sub-folder and in there is the main theme CSS file (although it may be a .php file). This must have the same name as the theme so – using the above example – rename ‘default.php’ to ‘custom.php’.
3. Also in your new theme folder is a file names ‘spTheme.txt’. You need to edit the header details of this text file and also, optionally, specify the list of colour overlays (although you will inherit all of the overlays from the original theme).
4. To do things really properly – although recommended this is optional – you should also edit the spFunctions.php theme template to change the localisation name. Swap the references to ‘spDefault’ to something like ‘spCustom’ (following our example).

That worked like a charm! Thanks!

Found out that you better not do these edits on an active theme. 😉

yes, correct… better to have your own – unless you never plan to upgrade it…