Yes, you are completely right. Because of the transparency, the code needed to set the color, uses a little bit differet color system, namely rgba (more info on that you can find here under the RGB and RGBA headings).
Currently this is the code which sets the background color of the sidebar menu:
background-color: rgba(247, 245, 231, 0.7);
If you don't need transparency, than you can change the rgba to the hex value settings, so for example if you would like it to have the same color as the main menu bar, you could use this code:
I hope this helps you. :-) If you have any more questions, just feel free to ask them here and I am happy to help you.
Toth Balint BT