Vague block focus in email composer

[sefsh]

Description

Steps to reproduce

1. Create an email activity
2. Start adding blocks in the composer
3. Add a block
4. Click on the Tune button

Expected Behaviour

Clear highlighting of what is being "tuned".

Something like this.

background: #e1f2ff;
  border: 1px solid #9ed5ff;
  border-radius: 5px;

Actual Behaviour

Vague highlighting of what is being "tuned".

[richardolsson]

This issue should be revisited with the new editor being introduced in #2476, and when that happens it's a good idea to look at @ajoensson's fix in #2481 which contains styling that can be reused in the new code.