how can i change background color ?

Hi arul,
you will need to add some custom CSS to your site.

If you log into your site and go to appearance -> customize, and then select “Additional CSS” you can paste the following code to change the colour.

.hdq_question {background-color:#000000 !important}

#000000 is the HEX value for black. You will need to change it to whatever colour you need.