Use CSS variables to set line highlight property values

[gadenbuie]

Hi @shafayetShafee! This is a small PR that I will improve the ergonomics for anyone wanting to customize the line highlight styles you set, without having to copy your CSS and rules into their stylesheets.

With this change, someone who wants to make the dimmed lines less opaque could add

:root {
  --lh--dim--opacity: 0.6;
  --lh--dim--opacity-hover: 0.6;
}

to their stylesheets. Then, if you ever need to adjust your CSS rules in the future, as long as you use the same CSS variables, they won't need to update their local styles.

Note: this PR shouldn't change behavior but you should definitely double check my work 😄

[shafayetShafee]

Hello @gadenbuie, Can you add some documentation regarding the CSS variables in the README?

Anyway, thanks for this PR. I feel flattered that you are contributing to this extension 😃 .