You can set the font size as a percentage (e.g. 105% would be 5% bigger than the default size). You can also use em, px, or pt.
If you want to use a different font, just replace whats in the box with what font you want (e.g. Arial) and install as normal.

Here's http://adf.ly/krZ2O you can click if you're feeling nice. Thanks!

Changelog:
05/04/2017
+ Improvements to custom colours.
+ Notices are now coloured.
+ Fixed custom link colour affecting bookmark numbers.
+ Minor fixes.

23/01/2017
+ Changed custom link colour to affect all buttons (instead of just most of them), for more consistency.
+ Made relationship tag backgrounds stop overlapping the dotted underline of the above tags, for better readability.

15/01/2017
+ Minor fixes.

21/10/2016
+ Fixed the close button on help popups.

28/08/2016
+ Black code/chatlog style text is now light grey.
+ Minor fixes.

09/08/2016
+ Added options to change background, font, and link colours! Works best if colours similar to the default of this theme are used (i.e. light font on a dark background).
- Custom colours won't affect everything, but most of the theme.
- Changing link colour also affects some buttons.

08/08/2016
+ Added support for https://greasyfork.org/en/scripts/5835-ao3-kudosed-and-seen-history and https://greasyfork.org/en/scripts/3144-ao3-kudos-hits-ratio userscripts.

11/07/2016
+ The tag category arrows in filters are now the same colour as text and easier to see.

18/06/2016
+ Fixed buttons looking strange after clicking on them and moving the cursor away.

17/06/2016
+ Fixed bookmark editing.

08/06/2016
+ Changed relationship tag background to be slightly darker and easier to read.

02/06/2016
+ Fixed font size breaking last update (oops).

22/05/2016
+ Fixed wrongly sized notes.
+ Minor fixes.

18/05/2016
+ Fixed previously visited links turning black when clicked/hovered on.
+ Made the color of text boxes slightly darker.
+ Minor fixes.

09/04/2016
+ Minor fixes.

06/04/2016
+ Fixed drop-down menus.

03/04/2016
+ Minor fixes.

26/03/2016
+ Fixed buttons.

08/03/2016
+ Fixed unread/unreviewed comments.
+ Fixed statistics graphs.
+ Minor fixes.

14/02/2016
+ Added option to change fonts!
+ Changed the font size option, so you can pick a more specific value.
+ Font changes also affect summaries while browsing fics.
+ Font changes also affect news articles.

14/10/2015
+ Fixed the border on form headers.
+ Removed white box on search suggestion.
+ Removed white top border on search text boxes.
+ Removed box border around help button.

02/10/2015
+ Fixed the weird things that happen when you click on headers.
+ Changed search box to same rectangle shape as buttons.

09/09/2015
+ Fixed font size.
+ Added smaller font sizes as well.

13/08/2015
+ Search results drop downs look better.
+ Made tables nicer.
+ Fixed External Work Bookmark page.

27/07/2015
+ Font size should work again (silly mistake on my end).
+ Tables should look better.

28/06/2015
+ Fixed history view count.

13/05/2015
+ Fixed leftover white borders.
+ Fixed Language and Tag buttons.
+ Minor fixes.

12/05/2015
+ Borders have been changed to grey.
+ Buttons also look nicer.
+ Removed thick bar from under icons in comments.
+ Fixed bookmark numbers not showing properly.
+ Fixed tags not showing when creating a work.
+ Fixed draft previews.

22/03/2015
+ Fixed bookmark box.
+ Minor fixes.

28/02/2015
+ Update for new homepage.

06/12/2014
+ Fixed relationship tag backgrounds.

07/10/2014
+ Font size changing has been redone.

23/09/2014
+ Fixed help button.

04/09/2014
+ Changed and added another option for changing font sizes.

13/08/2014
+ It is now possible change the font size!
+ Minor fixes.

11/07/2014
+ Fixed comments.
+ Minor fixes.

09/07/2014
+ Fixed help popup display.
+ Made red menu bar look neater.
+ Minor fixes.

06/06/2014
+ Fixed login page colours.
+ Minor fixes.

28/04/2014
+ Fixed bookmark box to be dark.
+ Minor fixes.

27/04/2014
+ Visited links now show up as a darker color instead of looking the same.
+ Minor fixes.

25/04/2014
+ First version released.