This is a bug with your theme as far as I can tell. I am observing the following:
As you can see, the computed font-size is 0px; which is an invalid value. Clicking the -> arrow in the inspector arrives at the following:
Disabling the entire CSS rule for
select, input, button, textarea, button like so:
Shifts the problem further down:
Finally, disabling the
font-size:inherit; rule on the
input, button, textarea, select, optgroup, option CSS class, and we end up with this:
Which is reflected by the text box itself:
It's worth mentioning that it's completely pointless to set font-size to 99%, as it is impossible for monitors to render part of a pixel. Setting "99% of 13px" results in 12.87px worth of text, which is a completely meaningless statement. You can either have 12 or 13 px, nothing in between. Why your theme would try to do this is beyond me.
At this point I am unable to explain why those two CSS statements fail only on the Shoutbox, and not on other textareas. There's no functional difference between the two, and the CSS is identical.
Sorry I couldn't be of more help
