<p>Currently, some of the default color combinations available in Bootstrap (such as the various <a href="../css/#buttons">styled button</a> classes, some of the code highlighting colors used for <a href="../css/#code-block">basic code blocks</a>, the <code>.bg-primary</code> <a href="..css/#helper-classes-backgrounds">contextual background</a> helper class, and the default link color when used on a white background) have a low contrast ratio (below the <a href="http://www.w3.org/TR/WCAG20/#visual-audio-contrast-contrast">recommended ratio of 4.5:1</a>). This can cause problems to users with low vision or who are color blind. These default colors may need to be modified to increase their contrast and legibility.</p>