ok figured it out if anyone else wants to know, you can achieve this by putting the following code into the initialisation function in the jhtmlarea js file:
//--- start inserted code-----
var cssLinks = $('head').children('link');
for(var i = 0; i<cssLinks.length;i++)
//--- end inserted code-----
Note: this includes all css from your current page, which includes backgrounds etc. you'll want to include a text only css file that deals only with p, h1, h2 etc.