Please check if this could help you.
If are okay with the idea, mention the color code within RGB() that you prefer. Under the Background color expression for each column.
I don't think there is a possibility to make the whole table background color changed in default table. You may have to look for extension.
In the Feb 2018 Qlik Sense Update one of the new features is the ability to
custom style your app better.
I would recommend waiting for the update.