On my board I found out that I need another CSS setting for the class .form-input
I have to add a width: 97%;
If you have a small browser window it can happen without width: 97%; that you get h-scroll bars while reding postings in your browser window.
After adding the css code to form-input the problem is gone. Its here on this board the same problem with narrow browser windows.
If you use a custom header it can be look nasty because it doesn't allign with the body when you resize the browser window to see the whole body.