add additional params background.bgColor to customize background

[hpcsc]

Motivation and Context

At the moment, the theme only allows to customize the background in 2 ways:

• set colorBG to true to randomize background colors from a predefined list
• set bgList to a list of images to randomize background images

For my site, neither of these options are what I want. I just want a simple color for the background color and not changing every time. So I updated the code to allow 3rd way of configuring the background color: by passing the color directly

Description

• I added another parameter background.bgColor to allow user specify the background color directly
• I also renamed the existing variable colorBG to randomizeBgColor to make the behavior clear. This is a breaking change but should be simple enough to update
• Both bgColor and randomizeBgColor set the same style property --background which will be used directly in css background
• Behavior of bgList is still the same

Please help to review my PR

[hpcsc]

please ignore this PR, I'll create a new one instead. This PR includes more than 1 change