Issues with the new default editor theme

### Tested versions

Commit [8afb2e5](https://github.com/godotengine/godot/commit/8afb2e59761cda17f23977c8ff2f4d57831796cc)

### System information

All applicable builds for commit 8afb2e5

### Issue description

**Issue Description:**

Commit [8afb2e5](https://github.com/godotengine/godot/commit/8afb2e59761cda17f23977c8ff2f4d57831796cc) was merged in today to change the default theme to a new theme that had only been discussed for a month among a limited number of members of the Godot GitHub contributors. This theme lacks many design elements that were functional in Classic Godot Theme and have been promised to be addressed. However, these issues were raised early on in the PR and were still not addressed.

Additionally, this is a large change to the default environment of Godot that was taken with little feedback and no proposals. When asked in PR #111118 just after the PR was created where this change was discussed, the question was left with no response. Changing the default environment theme (which would provide the baseline for how future themes are designed) should have more input than a month discussion of a limited number of contributors.

**Specific Issues with New Theme:**

[Scene Tree no longer has hierarchy lines in the new Default Godot Theme](https://github.com/godotengine/godot/pull/111118#issuecomment-3357650934)

[Docks no longer have a darker inner BG that visually separates contents from the dock border.](https://github.com/godotengine/godot/pull/111118#issuecomment-3357650934)
Specific Examples:
- Darker Scene Tree BG no longer exists.
- Debugger BG behind text no longer appears.
- Dropdown elements in lists are indistinguishable from other elements/BG (Object Inspector, [Input Map](https://github.com/godotengine/godot/pull/111118#issuecomment-3463004725), Create dialog, TileSet editor)

[Nodes in Node Editor blend into the background.](https://github.com/godotengine/godot/pull/111118#issuecomment-3470469761)

[No discussion/proposal before the theme was decided as the new Default Godot Theme.](https://github.com/godotengine/godot/pull/111118#issuecomment-3357623048)

**Proposed Resolution/Action:**

1. Revert PR #111118 (Commit [8afb2e5](https://github.com/godotengine/godot/commit/8afb2e59761cda17f23977c8ff2f4d57831796cc)) until a proper community discussion on a new default theme can be had, including multiple proposals for a new theme and not one selected for nebulous reasons.

2. Set the new default theme as an optional theme, and keep the current "Classic" theme as the default theme.

### Steps to reproduce

Read PR #111118 .

### Minimal reproduction project (MRP)

Read PR #111118 .

Start by reading PR #111118 and commit 8afb2e5, then compare the new default theme with the Classic Godot Theme using the listed Scene Tree, docks, debugger, dropdown, and Node Editor examples. The issue has no named files or tests; completion would require a resolved direction on whether to revert the change or address the reported visual differences.

Written by the indexing model from the issue text.