well, interfering is not the word I would use… 😉 you are displaying them in the forum, so natural to assume the forum styles would dictate appearance… but you are right, the po.st css is so generic that the more specific forum styles are being used instead… would have been better if po.st had been specific so they would know their styles would get used… I could see them getting overridden in many wp themes too…
so a few choices of how you want to proceed…
one, try removing the reset css in the forum styles of the theme you are using… its an include so you can just comment it out… with the reset css out of the way, you may get them to show… however, without the reset css, you may see the wp theme styles now overriding some forum styles… could be catch 22…
two, copy the po.st css file and add it to your sp css and add the specific forum container to the various styles such that they are more targeted… the css file they use is at: http://i.po.st/static/v3/css/r1post.css?3_31_2_pch_601 downside to that is if they change it or you change how you invoke it, you might not stay current…
do some detective work with a good css inspector like firebug and figure out which reset css files are controlling the display… and only add those (probably a handful) ones that you need to override the forum css with in your sp theme…