MDL-71385 theme: Fix contrast for dimmed text against striped table rows
In order to achieve good colour contrast between dimmed text and striped
table rows, the background colour of the striped rows are being
lightened to 0.03 alpha. While $gray-600 is being adjusted to a bit
darker shade of #6a737b. This gives a colour contrast ratio of 4.5:1
between the dimmed text colour and the striped table row background
which meets WCAG 2.1 Level AA requirements.
Comparing the normal font colour (#212529) with the adjusted dimmed text
colour gives a contrast ratio of 3.20:1 which meets WCAG 2.1 Level AA
requirements and still makes them distinguishable from one another.