Fix transparent panel in Tilset polygon editor using Classic theme

[Giganzo]

Fixes: #112326

Before:

After:

4.6.Dev2:

[Giganzo]

Classic theme was missing the stylebox for

godot/editor/scene/2d/tiles/tile_set_editor.cpp

Line 373 in 0fdbf05

expanded_area->add_theme_style_override(SceneStringName(panel), get_theme_stylebox(SNAME("expand_panel"), SNAME("TileSetEditor")));

[kleonc]

Classic theme was missing the stylebox for

Out of scope of this PR but maybe it would make sense to add some kind of auto detection / test for mismatches between different Themes to avoid something like that in the future.

[Repiteo]

Thanks!