Localization: window tab icons for "Add Locale" and "Add Custom Locale" render at too low a contrast to read in the Dark theme, unlike every other window tab icon in the same layout

Steps to reproduce:

1. Create a new project
2. Install the Localization package via "Window > Package Manager"
3. Confirm the Editor theme is set to Dark in "Unity > Settings > General > Editor Theme"
4. Open "Edit > Project Settings > Localization" and create the Localization Settings asset if prompted.
5. Click "Add Locale" in the "Available Locales" section to open the "Add Locale" window.
6. In the "Add Locale" window, click the three-dot menu and select "Add Custom Locale" to open that window as well.
7. Undock both windows so their tabs sit alongside other Editor window tabs, for example, "Project Settings", "Package Manager", "Animation" and "Search".
8. Compare the icon on the "Add Locale" and "Add Custom Locale" tabs against the icons on the surrounding tabs.

Actual results: Both the "Add Locale" and "Add Custom Locale" tab icons are drawn in a tone close to the tab background and do not read as a recognisable shape. In the attached screenshot, nine window tabs are visible at once: the icons on "Project Settings", "Package Manager", "Animation", "Search", "Hierarchy", "Project", "Inspector", "Scene" and "Game" are all legible, and the two Localization window icons are the only ones that are not.

Both windows also use the same generic pin marker instead of a Localization icon, so even at readable contrast, the tab doesn't identify the window.

Expected results: Tab icons on "Add Locale" and "Add Custom Locale" use the same contrast as other Editor window tab icons in the Dark theme.

Reproducible with version: 1.5.13 (6000.6.0f1)

Tested on (OS): MacBook Pro - Tahoe 26.6.2