Some of our themes such as Mute and Japan allow for you to choose a solid color as a background in your sidebar menu.
To start click the 'Edit theme' icon on the top right of your logged in home page to open the edit side panel.
Look for the following panels:
'Sidebar color' or 'Menu background color' - this is where you can choose a color
'Sidebar content style' or 'Menu content style' - if you've selected a dark color you'll want the menu content to be 'light' so you can read it and vice versa.