How do I in WordPress change the widget sidebar background colour to the custom colour appearance I want?
To change the look of the widget background to a different colour you will need to adjust a tiny bit of code in your stylesheet css file. To do this go through Appearance > Editor in your WordPress themes Dashboard. You should always work on your WordPress child theme and not the original WP theme.
Then copy in the html code below into the child theme stylesheet css - that should appear automatically if you have created a child theme - and change the colour to the colour that you want the sidebar background colour to appear as.
margin: 0 0 2.2em;