I recently released the new 3.0.2 version of ZAD Style Switcher, which can be used in Contao 3.x.

This extension adds a style switcher to your website, for example, to allow the choice of the font size, or to provide alternative versions of the pages (high contrast, text only, etc.).

Of course, after installed this extension, you have to create the CSS stylesheets to achieve the desired effects.

The extension is in the Contao repository:
https://contao.org/en/extension-list..._switcher.html

User manual is here:
http://trinko.github.io/contao-zad_switcher/