Validated using the extension-samples branch documented at #299479 (comment)

@gjsjohnmurray can #311339 be affected by this? (SVG currentColor is not replaced by current theme colors, resulting in black unless color is set explicitly)
I can test something if needed.

@gjsjohnmurray did some testing of this PR, maybe I'm doing something wrong, but can't confirm colors fallback to defaults:

Icons assigned as TreeItem.iconPath = new ThemeIcon(codiconId)

Can be related to the issue - even some debug-* icons which are colored in window.createQuickPick() don't get colored when used in the tree view.

@n-gist is the source of your test extension available somewhere? I'm puzzled that symbol-* codicons aren't colored in the quickpick.

@gjsjohnmurray it is not released yet, but I can share current version. Do you need source code or vsix would be enough? Send it somewhere?
For quickpick, it doesn't supports custom colors, but theme icons are supported, at least those inferred from resourceUri #308704

@gjsjohnmurray sent an invite to https://github.com/n-gist/temp.vscode.focus

To test icons - open a project and drop some files/folders to the extension view, and their icons will be customizeable.
To be free from resourceUri effects - create a group item by Arrange > Group, and customize its icon instead.

Customize > Icon for edit the value used in new ThemeIcon(id, color), format - id or id, color, like heart, problemsErrorIcon.foreground.

Customize > Icon Select is similar but through quickpick built over a list of builtin codicon ids.

Open issue in repo for any needs, I would be happy to assist with testing/fixing icons issues.