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Speeds up the download time of the page - good for Google and good for users. Helps browsers cache objects and speeds up future page views and pages from the same site.
Look at the source code to see if you're linking to a CSS/JS document or displaying it on page.
Easy to change if it's on page - stick it in a new document and link to it. Google around for help.
Show the MPU (square) unit on the left side of the content, not the right. Just have one unit, not the double-decker. Remove the left sidebar unit. Pace sitelinks in the sidebar or top navigation. Have a wider unit at the bottom of the content page. Split test 4 different colour palettes to see what works best.
That's a good starting point.
It's all about experimentation and testing for results. There's no real golden answer - you have to find it out for yourself by seeing what works for your site.
Good luck.
(I'm a former Google AdSense employee).