I’m no expert on WP, but the CSS is very easy as long as you know what the ID/class of your menu is.
For me I have a #menu-primary – that is a UL with that ID. You want to effect the anchor (link) within the list item within that menu. In my style.css I have the line:
#menu-primary li a {padding: 5px 10px; margin: 0px; border: 1px solid ccc; background: #efefef; display: block; text-decoration: none; color: #0DA4D3;}
This adds padding of 5px above/below and 10px to the sides of each menu link, its also makes the links behave like buttons because of the block display. Something like: [__Home__][__About__]
Obviously there’s a lot more you can do like making the button change appearance on rollover – but I’m not sure how much HTML/CSS you know?
If you are completely lost – try right clicking the menu in your browser and clicking “Inspect Element”, this is how to find out which bits of the html/css you are wanting to edit, and finding out the ID name of this particular menu. Hope that helps.