Hi there!
You have background color changing option in the preset tab of your template options. You need to turn on the custom style button in order to get those options. Here is a screenshot for your ease:
https://prnt.sc/x8baz8
And yes the 'a' stands For alpha. The alpha value indicates transparency or opaqueness of that particular pixel.
Path towards Template Options:
Extensions>Templates>Styles>
Click on your current default template name then choose Template Options.
Note: You are supposed to not touch the template.css file. If you want you can create a custom.css file and keep all your custom css code inside.
Hope this helps!
Best Regards