Forumlar / Cotonti / Support / Problems with forum

<img src="https://lh6.googleusercontent.com/_5me2KZIfG40/TYzn5wcSqwI/AAAAAAAACA0/_C8qfjpur8U/s800/ForumProblem.jpg" 

Hello,

As you can see in the picture above, I am with this problem, and I already know he canbe solved here:

select[name="jumpbox"] {
  width: 15em;
}

But I am new in Cotonti and I wonder how I can change that
Thank you all for your attention

Sorry for my bad english