Thanks for the answer!
How can I replace .qa-nav-main-custom-5 my button class question?
I have found another solution:
Created css-style for the "Questions" button and put it with the plugin "Widget Anywhere" in the "HEAD" pages of questions.
For example (If you take your style):
<style>
.qa-nav-main-questions > .qa-nav-main-link {color:white !important;font-weight:bold !important;border:1px solid #004566 !important;}
</style>
I have everything working!
PS. I am sorry for my bad english :-)