The CSS is applied inline to the "post" div.
That is to say the HTML when viewed with Chrome Developer Tools looks like this:
<div id="post" style="width: 500px; position: absolute; top: 0px; left: 0px; " class="masonry-brick">
I don't know much about Tumblr. can you access that line of code?
Weird, using FireBug it seems that the only inline style being applied is width="500px".
Looking at the source code, this is all I could fine (line 247)
Removing that centres the text in FireFox.