Font and Menu adjustments on Mobile version

Hello,

I’d like to reduce the height of the header and the menu on the mobile version.
I’ve tried #access, #header and others but it didn’t work.

Furthermore, the font size of the excerpt and the post on the mobile version is small. Is there a way to change it using CSS? I’ve tried several ways:

#content p {font-size: ..;}
#content {font-size: ..;}
body #content …
.post p{..}

But no result.

Thank you.