Before:
some icons were not in the same grey as the others. It was not clear whether they are disabled or not (because of the light grey color)

After:

Now it is much more easier to change the brightness with filter: CSS rule. For example see the Flat theme:
Before (Flat theme):

After (Flat theme):

Changes proposed in this pull request:

• CSS
• SVG icons colored into the same grey

How to test the feature manually:

1. look around and enjoy the new clear look

Pull request checklist:

• clear commit messages
• code manually tested
• RTL CSS (will do it after the first feedback)