Applying custom CSS rules to the highlighted area about maphilight

As a workaround you can locate the canvas element and apply CSS directly:
http://stackoverflow.com/a/38147321/1693593

But I would agree - it would be ideal to have an option where you can set things like blend-mode or css rule(s) text string. Just note that the rule would be applied to the entire canvas element, not just the highlighted area. Since the background graphics doesn't exist on the canvas element there is neither the possibility to use context blend modes for the highlighted region without rewriting the plugin to draw the image onto canvas directly. But this would be up to the authors of course..