How do I change body font size and menu bar?

First of all, create a child theme with just a style.css file in it. You don’t have to, but I strongly recommend it.

Question 1:

Create a style rule like this in style.css (at the end if it already has content):

body {
  font-size: 110%; /* or whatever */
}

If this doesn’t work everywhere, you may need a more specific rule for some occasions. Ask again if this turns out to be the case.

Question 2:

Best do it in a child theme.

(not sure what you mean by changing fonts or how it relates to your question, sorry)

Question 3:

You need to change this rule, I think:

#access {
  background: -moz-linear-gradient(#252525, #0A0A0A) repeat scroll 0 0 transparent;
  box-shadow: 0 1px 2px rgba(0, 0, 0, 0.4);
  clear: both;
  display: block;
  float: left;
  margin: 0 auto 6px;
  width: 100%;
}

You probably need to add something like this to your style.css:

#access {
    background-color: #nnnnnn;
}

Where #nnnnnn is the colour you want.

Should do it, I think.

Cheers

PAE