Theme editor controls are unlabeled for screen readers.

### Tested versions

* 4.6.2.stable

### System information

Godot v4.6.2.stable - Windows 11 (build 26200) - Multi-window, 1 monitor - OpenGL 3 (Compatibility) - NVIDIA GeForce RTX 4070 Laptop GPU (NVIDIA; 32.0.15.9571) - Intel(R) Core(TM) Ultra 9 185H (22 threads) - 63.44 GiB memory

### Issue description

In the UI theme editor in the bottom dock, there are several unlabeled controls when using a screen reader. These include:
* All override items
* The tabs for override types
* The edit field after the overrides list in the tab order
I used the NVDA screen reader, specifically NVDA 2026.1.

### Steps to reproduce

* Create a new project
* Create a new theme resource
* Select the theme tab in the bottom panel
* Add a new type to the theme
* Use tab while a screen reader is running to reach the overrides section and
* This entire section is unlabeled
Note: Depending on where you are when you start pressing tab, you may get stuck in the multiline field in the preview section and may need to use other methods such as a mouse or screen reader navigation commands to get out of this field.

### Minimal reproduction project (MRP)

N/A