Audio block: Improve focus style

[carolinan]

Describe the bug
When the audio block is added, it looks different in different browsers as expected, but the outer focus style in all browsers needs to be more visible.

In Firefox, there is no spacing between the element and the thin dotted outline so the outline is difficult to see.
In chrome, the focus is thicker, but there needs to be more spacing, see screenshot below.

To Reproduce
Steps to reproduce the behavior:

1. Add an audio block. There is an audio file in the Theme Unit Test if needed
2. View the front
3. Place focus on the block
4. See error

Screenshots
Chrome at the top, Firefox below

[melchoyce]

Solid focus style:

outline-offset: 5px;
outline: 2px solid var(--global--color-border);

Alternately, dotted like buttons:

outline-offset: 5px;
outline: 2px dotted var(--global--color-border);

(Unsure if border is the right variable here, just knew it would get the right color when testing)