You can change the width of the whole website in the THEME SETTINGS.
Go to APPEARANCE -> THEME SETTINGS (if you didn't yet).
The checkout the GENERAL tab, there you will find a block of options like for the website width, container settings, typography and A LOT more ;)
The CSS tab is the absolute correct way, yes. Never change theme files as all your changes will be lost when updating.
Use the CSS tab for CSS, as soon you'd like to change more in the PHP, the proper way to go is creating a child theme.
@smartshot - 120% for the container means it "should" be 120% of your display width, that is not good actually. Always stay below 100%.
@anietienkana, do you use CC version 1.10 and WP 3.5?
In this case, please update from the latest Github version and check if the error still exists.
CC free 1.11 @Github - https://github.com/Themekraft/Custom-Community/
You can even switch to responsive mode now and have Custom Community responsive!
The update is submitted to the WP review team already and should be available here very soon, too.
** Note: unfortunately we just noticed some new bugs that came from adding responsive features. We're working on it and a fix is already on the way. So take care changing things that affect the content width with the new version until the fix is available..
Hope this helps.. :) Konrad