While @SimeVidas makes a valid point, I believe that his response was somewhat hasty. It's important to exercise caution in these matters. I have made some updates to his fiddle http://jsfiddle.net/fRpQ2/4/ to illustrate my point.
When a specific property is declared in the stylesheet for a particular tag, that value will not be inherited from the parent with the inline style. This may be the issue you are facing when attempting to style the forum post. Unfortunately, there isn't much you can do about this without using style-tags or linked stylesheets. It seems like a lot of manual copying will be necessary in your case. Alternatively, you could inspect the website and apply existing classes to your post, but only if you want to mimic their current styling which I assume is not your intention.
I recommend doing some research on the cascading order of styles if you're interested in learning more.