Tweak formatting of highlighting

[damithc]

v1.9.0

Current: bootstrap .mark style is a bit too light:

Suggestion: tweak it to be a bit stronger? Example:

[Chng-Zhi-Xuan]

The weaker highlight style is from Bootstrap 4, which is using the colour #fcf8e3.

A quick fix is to override this in markbind.css to a stronger colour.