MDL-69392 theme_boost: Darken red, blue and green colours
The current red, blue, and green colours achieve the minimum colour
contrast against a white background. However, when they're against
coloured background, text with these colours will easily fail the
4.5:1 contrast ratio requirement for Level AA.
The updated colours now have the following contrast ratio against a
white background:
* Blue - #0f6fc5 (5.12:1, from 4.58:1)
* Green - #357a32 (5.27:1, from 4.63:1)
* Red - #ca3120 (5.29:1, from 4.59:1)
