In the Configuration panel, when you click this icon: you arrive on the page Config Style.
Config Style :
On this page you can manually modify the CSS style sheets of the skin selected for styleplus.css, RD mobile, fontstyle.css and the specific style sheets of each subpage.
Warning: you can view but you can't modify the style.css and jquery.css sheet. To modify them you must do so in Config look.
As stated in the warning at the top of the page, if you are not familiar enough with CSS, it is best not to try risky modifications.
A tip in this case: it is better to stay at the possible changes by Config look or use a skin already developed.
In the font style sheet you will put if necessary the @font-face of the additional fonts. Each of these font definitions will be included in Config look for font selections.