Filling page menu bar 100% width

Hello,
I would like an 'empty' button at the right end of the page menu bar which fills the page horizontally 100%.

To explain, here is how it looks now:


I tried adding a background attribute to:
echo '<div id="menu1" style="background: #EEEEEE;">';

in bfa_header_config, but then ofcourse the white space between the buttons is gone:



Tried solving that with adding a white border to the buttons on the right side, but because of the space between the buttons this doesn't look nice:




I am sure that what i'm doing is not very 'neat'. But it was getting very late and I was desperately looking for a quick solution. Hopefully some of you came up with, probably a much easier, solution.

Thank you so much in advance.