[no-jira] Applied instructure styling to engagement index

@richcreationsrc shouldn't that be something like:

```
@include background(linear-gradient(#5bc0de, #2596b8));
```

Don't think you need the `to bottom` in this case. The `@include` should do the trick.
Also have a look at https://github.com/ets-berkeley-edu/calcentral/pull/565/files#diff-a5a9f32d06f62b6aba756d05cc46ffccR80 if it wouldn't work out.