Hi Phill,
There isn't a tool which will allow you to tweak or modify a skin without first having a good handle on HTML, CSS and the settings/options available in mojoPortal.
If you are good with HTML and CSS, any editor (even notepad) will work fine. At this point, it's all a matter of preference. Folks on my team use Eclipse, some use Notepad++, others use Sublime Text and some use Visual Studio.
HTH,
Joe D.