Transparent content background

Sorry, if it was asked several times before, but I can’t find any answer. Is it possible to make the background of the content and the side-bars semi-transparent I am working with Mantra 2.3.3. and I am just preparing a new homepage for a client. This is the actual adress: https://www.mein-demoshop.de It is just a page for testing, therefore there is no child-theme.