I have always thought that Gmail gives me the perfect level of control when offering "Default", "Comfortable", and "Compact". What if theme's could opt in to a 2-, 3-, 4-, or 5-step scale of spacing? You'd get the best of both world's: A quickly-customizable way to make a theme feel really different and portability between themes. Per-block margins and padding are terrifying for anyone thinking beyond a DIY user, more than 5 pages on a site, and/or a website that is built to last 20 years, rather than 1.