Unfortunately, this can not be done with CSS. The theme uses the bootstrap framework with a class that determines the width of each column.
To be honest, I’d recommend leaving it as-is. The design was made that way on purpose by a talented designer. Trying to use up all of the available space is a common design mistake that designers spend a lot of time pushing back on. In this case, the line length, line height and font size are carefully balanced to make the text more readable. It’s really best to leave these things alone unless you have a professional designer guiding the changes.