You'll have to determine which color variation on that theme you are using. In the Site Editor, when you click on Theme in the top navigation bar, you should see a set of circular theme color variation buttons. The one you selected will be highlighted in some fashion (e.g., a white border). Take note of your selected theme color.
Now, in your screenshot, below the "main.less" file, you'll see the color variation less files (variables.less, variables_blue.less, variables_green.less, etc.). Let's say you chose the blue color variation for your theme. Look in the "variables_blue.less" file for the footer background color less variable "@color-footer-background" and you should see the current color specification. For example, if the footer background for the blue theme variation is set to something like #404040 (a shade of dark grey), in the "variables_blue.less" file, you should see a line where the footer background variable is set to that color --> "@color-footer-background: #404040;" To change this to white, change the color hexcode from #404040 to #ffffff.
