This is version 1.0 of visibility plugin for WordPress. Download it: visibility plugin for WordPress
The purpose of this plugin is to aid those with visual impairment by providing adjustable font size and high visibility CSS.
Install Visibility by going to Plugins, Add New, Upload, Choose File. Select visibility.zip and OK, then click Upload. Click Activate to start using it. Then go to Appearance, Widgets, find Visibility and drag it to the Sidebar widget area.
Edit visibility.css in /wp-content/plugins/visibility if you wish to change the colours or the elements changed. The one provided uses a sledgehammer of changing absolutely everything.
My thanks go to @gwenhwyfaer for her debugging skills.
Font size adjustment does not work in themes that used fixed font sizes. Specifically, THIS DOES NOT WORK IN TWENTYTEN. Sorry.
This will only work if WordPress is in the root of the server. If it is not, you must edit visibility.js and change the path to the CSS file.