You can customize the way Reader renders, by editing a single file. You’ll need admin access, and a bit of HTML and CSS know how¹.

First, do this:

1. Find Safari.app in your Applications folder
2. Right click
3. Click ‘Show Package Contents’ from the context menu

Then, find this (you’ll probably want to save a copy of this in case you want to revert):

1. In the new Finder window, go to the Resources folder
2. Find Reader.html
3. Edit with whatever editor you’re comfortable with.

Then, make some changes and save:

You might be asked for your admin password when you save. I set text-align to left because that’s how I roll.

Before:

After:

Seriously though, if you don’t know what you’re doing… please don’t even attempt.