A longstanding problem with web pages is that fonts are not actually included in the page code. The page simply includes some text, along with an instruction in the CSS that tells the browser what font to use to display that text. But if the visitor's computer doesn't have that font installed, it falls back to a default, or works its way down whatever font stack is indicated in the CSS until it finds a font that it can use. The effect of this limitation is that web page designers are generally forced to rely on a small set of "web safe" fonts, fonts that can reliably be found on most visitors' computers.
Packaging fonts with web pages in a way that allows the site visitor to install the font on his computer, while possible, runs afoul of copyright protections. Most fonts, even those normally distributed with a new computer, are protected by copyright, and you don't have the right to make them available to others.
To address this problem, Google Fonts and Web Open Font Format (WOFF) files can be used. The font is not actually installed on the visitor's computer. It is used only for displaying text on your web page.
In Mars, you may add two Google Fonts and a WOFF file, and tell the skin which font to use for each text element, with the default being the selected regular font family. Decorative fonts can be difficult to read at smaller sizes, and are therefore best used only for page titles. You are likely to find that they look better when displayed with a larger font size than you would use for a regular font, and with normal weight, rather than bold. More traditional fonts may be suitable for all text.
To choose a Google Font, visit the Google Fonts page, and identify the font you want to use. Scroll down on the page to view suggested font pairings. You might want to use a more elaborate font for titles, and a simpler font for captions. Make a note of its name, and enter the name in the Mars settings. Mars does not support the use of Google Font multiple styles. If you see a font listed, and it indicates that there are multiple styles of that font available, when you enter its name in the Mars settings, the skin will use the basic style of that font.
There are now many hundreds of free WOFF files available. Search the web for free woff to find sources (some sites require registration). In most cases, the fonts are free for you to use on your website, even if the website is commercial, but the font designers retain copyright, so you don't have permission to redistribute the files directly to others. Often, these free fonts are provided in a variety of formats - WOFF, OTF, TTF, EOT, SVG, and so on. The format needed for Mars is indicated by a .woff file extension. In many cases, the download will be provided as a zip archive containing several formats - you need to extract the .woff file from that archive and use that one for inclusion in the skin.
The FontSquirrel site is a typical example, offering many free fonts. Find one you like, and then click on the name of the font (not on the Download link). Then look for a Webfont Kit link on the menu bar. There, you can choose the WOFF format. If a font is available in only one format, the Webfont Kit menu bar choice will be absent. In that case, you can download another format, like TTF, then use FontSquirrel's Webfont Generator to upload the TTF and convert it to WOFF - choose the Expert radio button after you've uploaded the TTF version. In short, it may take some exploration to find the WOFF file you need.
See Fonts » Album to include a Google Font and/or WOFF file in your album.