IGN++ has a number of options you can change.  Options are organized into tabs and sub-tabs.  Firefox and Chrome have different interfaces for changing options, but option names and tab configurations are generally the same.  Specific options and their purposes are listed on their relevant pages under the "Options" header.

How to access

By default, you can access Options from boards.ign.com by hovering over the "Boards" drop-down menu on the top right of the page.  In the menu, you will find a section labeled "IGN++ Controls".  This will contain a gear icon that you can click to configure your options.  The IGN++ Controls area may not be there if you've chosen to disable it on the Integrated Tools tab.

On Firefox

See this page.  Note that it is currently out-of-date.  You can press Control + Shift + A to open the Add-on Manager on Firefox 4+.

On Chrome

See this page.

How it works

Both browsers store their default preference values in defaults.js.

On Firefox

IGN++ uses Firefox's Preferences API and a standard preference window, just like most other Firefox extensions.  Because of this, it is relatively easy to view and modify your preferences without ever opening the options window.  Simply visit about:config and type "extensions.vestitools." into the filter field to see all IGN++ preferences.  Modifying preferences from here should only be done if you know what you're doing or if the options window is broken for some reason.  You can read more about about:config here.

On Chrome

Chrome's Options tab has a few more features than on Firefox.  There are individual controls for each preference to save, revert to the last saved state, and revert to the default state.  Each preference's input is validated.  If you close the tab without saving something, IGN++ will let you know.  There are global save, revert to last saved state, and revert to default state buttons that act like their individual counterparts but apply to all preferences.  The Options tab has a lot of built-in documentation.  There is a "Show Hidden Preferences" button under Miscellaneous > Dev which will show preferences that would have to be altered through about:config on Firefox.

IGN++ uses its dedicated local storage to store preferences in JSON format.  The actual interface and framework around this basic concept is entirely homemade.  At its most basic level, it is doing something similar to what is shown in this example.  If you want to change preferences without going through the Options tab, the easiest way is to inspect IGN++'s background page and use the developer console to call GM_setValue.  This requires a bit of JavaScript knowledge - I think it goes without saying that you could easily break things if you don't know what you're doing.