Refactored the colors palette a little more

- Introduced text-dark-grey = #666666
- Converted text colors #777777 and #555555 to text-dark-grey
- Removed header-grey in favor of text-grey
- Included colors in a couple of files where it was missing
- Categorized the colors in the palette based on the contexts of usage
Merge pull request #4479 from ssrihari/fix-colors-more

Refactored the colors palette a little more