I recently created a new tumblr and grabbed a theme, but after some modifications (mostly after adding a corner image), my tumblr posts began covering the sidebar.
To view the code I used, please click here. Right click and select "view page source". I can't for the life of me figure out what's wrong, so any help would be greatly appreciated! Thank you very much!
Your sidebar div is fixed to the left, and your #posts div has a margin-left of zero. So they are both set to be aligned to the left of the page and since the sidebar is fixed, they overlap.
Setting the left margin of the #posts div to something like 400px will fix the problem (I tested this in the web inspector of my browser):
margin-left: 400px;