Customizable style for borders

### Feature Idea

It’s sometimes hard for me to see which node is currently active.
I’d like to make the green border around it thicker so it stands out more.

This should also apply to the red and purple borders around a node when they indicate an error.

### Existing Solutions

_No response_

### Other

_No response_

No files, tests, or entry points are named. Start by locating the frontend code that renders node borders and handles active, error, and other node states; review existing style customization patterns. Done means users can make the green, red, and purple borders visibly thicker without changing their state meaning.

Written by the indexing model from the issue text.