Add green outline to New Branch button

## What does this MR do?

Add green outline to New Branch button on issue page.

## Are there points in the code the reviewer needs to double check?

Nope.

## Why was this MR needed?

The button was not very visible.

## What are the relevant issue numbers?

fixes #18928 

## Screenshots

### Before

![before](/uploads/3bc890e249ef4c3852605a6350c05b9a/before.png)

### After

![after](/uploads/bc627902ffa7c7b4ab20ee7a269fa44f/after.png)

See merge request !5447