I think it has something to do with the comment form, but I haven't figured out how to fix it.

Have you managed to solve this yet?

¥

Yes, I removed the CSS for the comment form, and everything is now cool. Thank you for asking!

No problem ;)

¥

sorry, I have the same problem, but I don't know where is the CSS that I must remove :(
Can you help me, please?

It's this section of the CSS file:

/* styles for comment and profile forms */
form {
margin: 0;
padding: 0;
}
form.inline {
margin: 0px;
padding: 0px;
display: inline;
}
fieldset.input {
margin: 0;
padding: 0;
border: 0;
}
fieldset span.notes {
font-size: 80%;
color: #999;
}
input.large, textarea.large {
width: 50%;
margin-left: 0;
margin-right: 0;
}
label.radiooption {
margin-right: 1em;
} 
/* 24/75% forms - Fast Forms */
form.fform fieldset {
border: 1px solid #ddd;
margin-bottom: 1ex;
background-color: #fff;
padding: .5ex 1ex;
}
form.fform fieldset fieldset {
border: none;
margin: 1ex;
padding: 0;
}
form.fform div.label {
float: left;
width: 24%;
text-align: right;
font-weight: bold;
margin: 0;
padding-top: .5ex;
}
form.fform div.input {
margin: 0 0 0 25%;
}
form.fform fieldset.input {
margin: 0;
}
form.fform div.info {
margin: 0 0 0 25%;
padding-top: .5ex;
}
/* 18/80% forms: comments and profile */
form.bComment fieldset {	/* one form line */
border: none;
margin: 2ex 0ex;
padding: 0ex;
font-size: smaller;
font-weight: normal;
}
form.bComment fieldset label {
font-weight: bold;
font-size: larger;
}
form.bComment fieldset div.label { /* label of form line */
float: left;
width: 18%;
text-align: right;
padding-top: .5ex;
font-weight: bold;
}
form.bComment fieldset div.input { /* input zone of form line */
margin: 0 0 0 20%;
}
form.bComment fieldset.input {
margin: 0;
}
form.bComment fieldset div.info { /* info zone of form line */
margin: 0 0 0 20%;
padding-top: .5ex;
} 

/* input.bComment, select.bComment, textarea.bComment {
width: 50%;
} */ 

I just wish I knew precisely which part of this section was causing the problem. Removing it all fixes it, however.

great! thank you very much! :-*