I suggest taking a look at your html sanitization settings make sure html isn't allowed in markdown settings.
Next step would be to move the other 2 topics from that category and check if you still see broken markup.
I've selected Dark skin then rebuild and restarted the forum. Now even if I revert to default skin, It's using Dark as default skin. can someone help me with this?
@srujan where are you selecting the skin change, on server or client side?