In a theme you could probably do this by adding a setting that saves to localStorage, the downside being that users would have to change the preference in each browser they use.
You could have multiple themes, but that would get complicated if you want to offer more themes that change styles beyond the font.
A plugin would be a little more complicated, but would probably be the most seamless way to do it, because you could add actual settings instead of relying on browser storage or having many themes.