If you are moderately versed in CSS, you can use the Safari extension Quickstyle to select any page element and modify any of it's CSS attributes.
Once installed, click on the toolbar icon, hover over different elements on the page (they will be highlighted), and then click on the element you want to modify. A pop-up CSS editor will come up and you can edit to your hearts content. All changes are persistent, but non-permanent and you can edit or delete them as desired.
Note, I think the changes are stored in a cookie and will be wiped out if you clear cookies on Safari.
Changes can be made to the whole page or to specific elements site-wide.
I use it to increase the fonts for easier reading on a variety of sites, as well as to remove distracting elements.