(Adapted from the WC3 guidelines)
Providing access to content includes enabling users to configure and control its rendering.
Users with low vision may require that text be rendered at a size larger than the size specified by the author or by the user agent's default rendering. Users with color blindness may need to impose or prevent certain color combinations.
User agents with speech synthesis capabilities need to allow users to control various synthesized speech rendering parameters. For instance, some users may not be able to make use of high or low frequencies; these users have to be able to configure their speech synthesizers to use suitable frequencies.
Color blind and low vision persons can only see certian colors or see some colors better than others. By leaving it up to the client to choose the colors he/she wants to view on their browser is easy to do.
It's pretty easy. All you have to do is change the font and background tags of your document from author defined colors to operating system's color settings. Below are examples and images to help web page designers understand how to do this.
<font color="#000000"> or <text="000000">
Change the default of white (000000) to windowtext as shown in example 2.
<font color="windowtext" or <text="windowtext">
The above example shows that the web page designer wants the text color in the document to be the same as any end user's operating system's color. This is easy to do if the web designer used templates: to change the so all you have to do is go into the template and change this for all your pages that you need.
Do the same for the background colors and you're done.
* For an expanded explanation on this procedure, view W3C Guideline 3: Use markup and style sheets and do so properly.