[Fixed][Bug][Editor] Background color in the terrain/object bar fails to update

[IchoTolot]

When loading different themed levels, it happens that the background theme in the terrain/object bar fails to update properly upon loading a new level.

Examples:

Loading a L2_shadow level and still having a pure black bg in the terrain/object bar.

Loading a sports or snow level while still having the L2_shadow.

For reproducing it works the following way for me: 

- Load a level
- Load a level in a different theme that has another different single colored background
- The terrain/object bar should still have the 1st level's theme

[Nepster]

Fixed for the next version.