Annotation highlight too faint on e-ink

The default highlight style in `annotator.min.js` is `rgba(255,255,10,0.3)`, whick looks fine on LCD screen but is invisible on e-ink due to low contrast. The fix should be as simple as adding the following css to override it, something like `rgba(0,255,0,0.4)` helps. It would be great to have this option in the settings.

```
.annotator-hl {
background: rgba(0,255,0,0.4) !important;
}
```