Interesting. Those icons did not change between rc3 and rc4. In fact, they haven't changed significantly since rc1 when we switched from bitmap ICO's to scalable vector graphics.
What it looks like to me is the icons were reloaded as part of a switch to the dark theme, but then somehow, not reloaded again when you went back to the System theme. We adjust the image darkness for the themes.
Try this: go to Tools > Options > Appearance > Tool Window and Toolbar Options..., and change the style to "Monochrome", hit Apply, then change it back to "Default (Low Color)" and hit OK. This will force all the toolbar buttons to be reloaded. They should look exactly the same as they looked in RC3.
If my theory is correct, then the question is how the Dark theme options and the toolbar button loading got out of sync. We reload the buttons when you switch themes in the GUI. Would have to try to track down how your editor got into this state.
If this does not improve the appearance of your toolbar buttons, I can look into tweaking the image processing to darken the default buttons a bit more for light backgrounds.