Customize underline text attribute

In xterm it is possible to specify custom color for underlined text and disable underlines at all, for example with settings like:

```text
XTerm*colorULMode: on
XTerm*colorUL: yellow
XTerm*underLine: off
```

I can make underline text to be drawn in yellow.
Is it possible to achieve the same behaviour in blink shell?

Thanks

The issue names no source file, test, or entry point. Start by locating Blink's terminal rendering and configuration handling for underlined text, then compare the requested color and underline settings with the existing behavior. Done means custom underline color is supported and underlines can be disabled.

Written by the indexing model from the issue text.