[3.0] Primary button colour contrast is not high enough

[robbieaverill]

Starter 3.0.x-dev

Example:

Axe report:

Element has insufficient color contrast of 3.97 (foreground color: #ffffff, background color: #007bff, font size: 12.0pt, font weight: normal). Expected contrast ratio of 4.5:1

cc @silverstripeux

[robbieaverill]

This is an issue with Bootstrap 4 - see twbs/bootstrap#25126

@clarkepaul suggested using the old theme colours for the parts that don't meet the contrast requirements.