Add custom color for sub menu

[komawoyo]

Describe the proposed feature

Add ability to add custom menu color similar to main menu.

Explain how the feature may be useful

In the image below the colors are not linked so it does not look as good.

Screenshots

[bonzibudd]

Thank you for the feedback! I will definitely consider this as an option for future releases.

[bonzibudd]

Addressed #170 in release 1.5.3.