Current "themes" system only supports global background and foreground color

[danielweck]

See:
#203 (comment)

So, right now hyperlinks are color-inverted because of an internal / architectural limitation:
https://github.com/readium/readium-js-viewer/blob/develop/src/js/ReaderSettingsDialog.js#L14
(previewStyle.backgroundColor and previewStyle.color)

There would be noticeable improvements in readability if color themes described when major elements (such as hyperlinks) should be styled differently than the rest of the HTML document.

[danielweck]

Related issue: readium/readium-js#187