little question using mx4 theme

Operating System:windows 7 32bit 7601
Icedragon version:26.0.0.2

when I using mx4 theme, or FXopera theme , I found when after maximize +restore the window, there are abnormal small bar on each side of Window, it seems like some part of webpage, I tried in firefox and it is ok,and this kind of situation happens also in FXOpera.
the add on is at here:

https://addons.mozilla.org/en-us/firefox/addon/mx4/?src=api
https://addons.mozilla.org/en-us/firefox/addon/fxopera/?src=userprofile.

I don’t know whether it is the issue of Icedragon or the theme, or some draw routine issue, in firefox it is normal,hope a better Icedragon,thanks.

[attachment deleted by admin]