The Footer Sidebar Section of the customizer deals with the sidebar area located just above the footer of the theme. There are three sidebar areas in the Footer Sidebar and they are First Footer Widget Area, Second Footer Widget Area and Third Footer Widget Area. This sidebar area will only show if you add a widget to one the widget areas.
There is only one subsection of the Footer Sidebar Section and that is Footer Sidebar Content. There are two settings associated with this area. The first setting allows you to change the width of the Footer Sidebar Content area. Just like the Header Content section, the default setting is
64rem, which is equal to
1024px wide. You can change this value to whatever you wish*.
The second setting deals with the background color of the Footer Sidebar Content area. The default setting is
#ffffff You can select any color you want to use for the header section.
Both settings are dynamic, meaning you can see the changes you make on the right hand side.
Once you have made any changes to this area click
Save & Publish
*I recommend that you always try to use rem values. There is a great tool that will help you with calculating rem values called Rem Calulator.