CSS after upgrading MyBB

[.m.]

global.css - around line 442

.trow1 .smalltext, .trow2 .smalltext {
    width: 80%;
    float: left;
    padding: 3px 0 5px 7px;
}

you can try removing width & float properties by commenting out (like below)

.trow1 .smalltext, .trow2 .smalltext {
   /* width: 80%; */
   /* float: left; */
    padding: 3px 0 5px 7px;
}

remember hard refreshing your browser [eg. press CTRL + F5]
on member registration page to check the style changes ..