Thinking of changing a theme for my site on my own

[ClicketyKat]

I have a free theme for my forum that's PhpBB based and I'm thinking of just changing it on my own. Is this something I shouldn't be messing with? Do I need permission from the original designer of the theme? And what problems could arise if I just change the colors to something more pleasing?

 

[Kane]

I do not want to mess with phpBB. I once tried to add ad management extensions and suddenly my site started showing an error. The site was inaccessible for days until I had to restore the database. If you only want to change, that's find but if you want to do some editing, make sure you have the right kind of skills.