How do I change the Blog post bg color?

To change it try adding the following CSS code in the Custom CSS option of your theme on the below path.

Admin Area -> Appearance -> Customize -> Additional CSS

article.post {
    background-color: #FFEB3B;
}

You can change the color value in the above code to whatever you want to use by referring the following pages.

https://www.w3schools.com/html/html_colors.asp
https://www.w3schools.com/html/html_colorvalues.asp
https://www.w3schools.com/tags/ref_colorpicker.asp