Alternative CSS

<link rel="alternate stylesheet" href="path/to/style.css" title="yourTitle">

Some browsers allow alternate style sheets to apply if they are offered. By default they will not be applied, but usually they can be changed through the browser settings:

Firefox lets the user select the stylesheet using the View > Page Style submenu, Internet Explorer also supports this feature (beginning with IE 8), also accessed from View > Page Style (at least as of IE 11), but Chrome requires an extension to use the feature (as of version 48). The web page can also provide its own user interface to let the user switch styles.

(Source: the MDN Docs)