https://getfirebug.com/wiki/index.php/CSS_Panel
To know where the CSS is set you should first look thru and get familiar with all the theme options. Next you will need to look at the CSS Selector then edit the theme file css.php and search for that selector. In there you will find things like
HTML Code:
body {
text-align: center; /* centering the page container,
text-align will be reset to left
inside the container */
margin: 0;
padding: 0;
<?php echo $bfa_ata['body_style']; ?>
}
the line
HTML Code:
<?php echo $bfa_ata['body_style']; ?>
points you to the body style options at ATO->Body, Text & Links->Body Style
while it may seem confusing at first, I think Flynn (the developer) did an elegant job.
don't forget that you can always add CSS in the ATO->Add HTML/CSS Inserts->CSS Inserts option which will be at the end of the CSS and override any options you don't (or can't) change in the regular options.
p.s. there is no CSS in the default theme options for HTML so unless you added it, it is probably from a plugin.