highlight mathjax via setting background

[beepmo]

Currently, we use the latex \color command to make MathJax render our highlights in colour. This is bottlenecked by MathJax's queue implementation.

We could instead highlight by setting background, mathItem.parentElement.style.background = 'red'.

Considerations:

• use latex to highlight search text, which might be part of a large table
• we will need to put \color brackets around the smallest syntactically correct latex element that includes the user input

[beepmo]

find a smaller parent element!