Nirvana main menu items align justify

[ Moderator note: moved to Fixing WordPress. Please do not use Developing With WordPress for these topics. ]

Working on a Nivana theme, I would like the main menu items to be justified to the width of the page (the content width). I have tried with a few code snippets found in forums, but they do not work. Last one was this:

#access > .menu > ul {
display: flex;
justify-content: space-between;
flex-wrap: wrap;
}

But it does not change anything.

I wouldbe very happy if someone could helt.

Best regards
bibber