You already have a tool on your PC - it is named "notepad.exe"
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Having a graphical tool to edit the themes would mean limiting the possibilities what can be done by theming, leaving your design options limited (similar to the Nuke-like CMS apps that all look the same imo). There have been generations of desktop apps that tried to implement a graphical HTML editor, but there is none that easy both newbie-proof and powerfull as well (it's either newbie-proof and limited or powerfull, but hard to learn). There would of course be some users (like you) who would appreciate an easy tool for minor edits, but after all Coppermine is not aimed at HTML-virgins in the first place. If you don't know nor understand HTML/CSS, that's fine - you're of course welcome to use Coppermine anyway. However, our main focus lies on site owners who know their way around in HTML and CSS.
Additionally, someone would have to code such a tool. I doubt that a dev team member would be ready to do this, as it would take an enormous amount of time and would only lead to a crippled interface (compared to what can be accomplished by modifying the HTML and CSS code).
Therefor, I'm marking this request as "Turned down". Hope that you nevertheless enjoy using Coppermine.
Joachim
