Currently, contrast themes are applied by adding a class to the body which applies the appropriate theme from Enactors.css. For example, applying the black and white contrast theme assigns the .fl-theme-bw class to the body, which sets the following CSS custom properties:

These two variables are used within UIO's contrast theme stylesheet, which is applied to the adjusted website.

This works but has a significant drawback. Because the contrast theme stylesheet uses !important quite liberally, some of the website's styles will be forcibly overridden and will require further use of !important and lots of duplicated styling in order to properly integrate UIO into a given website's design language. Here's an example: https://codepen.io/greatislander/pen/ExZjpBW

In this case, a custom focus style has been applied to links which inverts the background and text color when the link receives focus. But UIO applies a background and text colour with !important to any element within the adjusted body:

 In terms of specificity, this overrules the theme's a:focus rule, even when the background-color property is given the !important flag. The following CSS must be added just to ensure that the custom color and background-color of the link focus style will be honoured when a contrast theme is applied:

In my experience, many such overrides of overrides are necessary when integrating UIO.
 
A different approach that would also have the impact of reducing the CSS needed to apply a contrast theme would be for UIO to provide only the relevant custom properties. In other words, applying the yellow on black theme would only add the .fl-theme-yb class to the body and apply the following styles:

Then, the integrator could use these two CSS custom properties throughout their stylesheets with a fallback to their theme's default colour. For example:

In this way the contrast theme provides colour variables which are consumed by the integrator's stylesheet, rather than attempting to override large portions of the integrator's stylesheet which the integrator may have to struggle to restyle appropriately while respecting the contrast theme.

There would also be significant benefits in terms of file size. The most complex contrast theme is this one:

Minified, this is 204b compared to the current contrast theme stylesheet's 20.2kB.

An important caveat is that this approach would necessitate some more comprehensive documentation on how to integrate contrast themes, and would require more deep integration into a site's stylesheets. This would have the benefit of encouraging integrators to write more flexible, adaptable CSS.