In a DM on Twitter Milind Alvares sent me this link after I asked him about the availability of a particular Safari Extension. Install all of these extensions because they really do make things a lot better looking. I also love the Reddit Minimal extension and the Pure Reader extension.
If you are not into extensions that re-design a website (which is fare) then just install the Beautifer and Optimize Legibility plug-ins that he lists. I use the font-smoothing commands in my CSS files and it really makes text look great in Safari.
Bonus Tip: Open ‘Font Book’ on your Mac > scroll down to the Arial font faces > right click > choose ‘disable Arial family’. Now any webpage that calls for Arial will most likely be rendered with Helvetica – you are welcome.