As Mario suggests, there are two ways to use Google Fonts with TW5: linking or embedding. Linking carries the consequence of creating a dependency between the TiddlyWIki document and Google's online font service. Embedding is more resilient in that it works offline, but carries the disadvantage of swelling the TiddlyWiki file with the font data.
The built-in 'Starlight' theme includes an embedded Google font called Arvo:
There are no special difficulties in doing this beyond the steps needed to create a theme plugin. Alongside the binary WOFF file containing the font one must create a .meta file that specifies the title and type of the font tiddler. You can see this in the Starlight theme:
Using the font requires several steps within the stylesheet. First you need this datauri macro:
\define datauri(title)
<$datauri tiddler="$title$"/>
\end
Then you need a @font-face declaration for the font. This is copied from the Google font information page:
@font-face {
font-family: "Arvo";
font-style: normal;
font-weight: 400;
src: local("Arvo"), url(<<datauri "$:/themes/tiddlywiki/starlight/arvo.woff">>) format("woff");
}
Finally, you need to use the new font in a font-family declaration:
html body {
font-family: "Arvo", "Times";
}