Theme icon colours

It would be nice to have the ability to choose an icon colour for blocks, that is different to the main content icon colours.

Our scenario is that we have dark blue side block headers and the default icons are grey - these colours don't compliment each other and the icons can hardly be seen... on the other side of this, the main "weeks" (course content) are a very pale grey/nearly white (background) and the default grey icons look very nice.

As the icons are SVG is there a way we can "on-the-fly" change the icon colours for certain blocks? I.e. white on a blue background, dark grey on a light background?