I see where I can change fonts, background colors etc,. for the main page menu, but how can I add space within the menu? (In this case I want to add space to the left of the menu so that it aligns with the other elements on the left side of the pages, and add space bewteen menu items.) http://lafollettegallery.com/home/

In general, what's the best way to go about making html/css edits outside of what's made available in the theme options?

Thanks in advance for any help.

I suggest that you use Firebug to see the ID's or classes of each area you want to edit, then try the change on the fly inside Firebug, finally add it at Atahualpa Theme Options -> HTML/CSS Inserts -> CSS Inserts

The page menu bar looks like this

<div id="menu1"><ul id="rmenu2">

So to move everything to the right you'd add a CSS Insert

div#menu1 {
margin-left: 100px;
}

or

ul#rmenu2 {
margin-left: 100px;
}

The div#menu1 has no styling so the effect will be different compared to moving ul#rmenu2, depending on other styling you might have on the page menu bar

Space between single <LI>'s

ul.rMenu-hor li {
margin-left: 30px;
}
ul.rMenu-hor li ul li {
margin-left: 0px; /* reset left margin for sub menu items */
}