Robert Chapin

Input fields on the WordPress admin screens have been nearly invisible since version 2.7. The problem was so severe that Matt suggested I make a plugin to override core CSS values for anyone who wanted more contrast. Following community feedback, I created this extra plugin specifically for high contrast.
This plugin adds Admin Color Scheme options that dramatically change the appearance of form fields, table rows, and background colors in the admin pages. It is not intended to follow the design principles of the built-in style sheets. If this is too much for your needs, then also consider my plugin with more subtle effects called <a href="https://wordpress.org/plugins/fix-admin-contrast/" rel="ugc">Fix Admin Contrast</a>

High Contrast Admin

Adds admin color schemes for the visually impaired.

fiskeben

This plugin adds a simple way to let your readers enable or disable a high contrast stylesheet for the visually impaired.
Once a reader enables the high contrast stylesheet the setting will be stored in a cookie which means the setting will not only stay one when navigating to other pages but it will also be on when the reader returns.

Vividly

Enables toggling an extra stylesheet with high contrast CSS for visually impaired users.

High Contrast Admin vs Vividly