Recently, Stack Exchange decided to switch to system fonts. For those of you who prefered the old appearance, or would like a different font to appear in code blocks or across the site as a whole, Custom Fonts is a user script I wrote this morning to allow just that.
Custom Fonts allows rules to be set for different sites, which can specify a sans serif, serif, and monospace font. A default can also be specified, allowing you to choose between sans serif and serif for any site.
Custom Fonts initially doesn't change anything (unless you download one with a preset), but on lines 26 and 27 are rules that revert all of Stack Exchange to its original fonts (once un-commented). Custom rules can be written, and there is a short reference in the comments under the
Custom Fonts is a user script, meaning that it requires Greasemonkey (Firefox or browsers that support Firefox extensions) or Tampermonkey (Chrome or browsers that support Chrome extensions) to be installed.