Short answer: FogBugz 8 no longer supports the ability to add custom entries to the Format, Font Family, and Font Size dropdowns in the wiki editor.
Longer answer: One of the biggest goals of FogBugz 8 was to give our customers a much-improved wiki experience. As part of that effort, we replaced our own bug-riddled wiki editor with a customized version of CKEditor.
Switching to CKEditor has not only eliminated the vast majority of browser bugs our customers had to deal with in FogBugz 7 and earlier, but has also enabled our customers to edit their wiki pages' HTML source directly (if they so desire). This has been a long-standing feature request and gives our customers a great deal of power when it comes to editing their wiki documents.
An additional benefit of moving toward a third party wiki editor is the ease with which we can apply updates to it as the CKEditor team updates their editor to support new browser features and fixes bugs. We have customized CKEditor in a number of ways in order for it to integrate more seamlessly with FogBugz; however, the more we customize it, the more difficult it becomes for us to apply updates to CKEditor that come from the CKEditor team. Custom fonts and formatting styles is one place where we have decided not to customize CKEditor.
The decision to lose support for custom formatting options in the wiki editor was not made lightly. This is why documents relying on these custom styles continue to look the way they did in FogBugz 7: we made sure to carry forward the custom font, size, and format styles into the new FogBugz 8 default template.
Workaround: The workaround for custom format styles (e.g., p.ExtraPadding {padding: 25px;}), is to edit the HTML source of your wiki article and manually add the necessary class attributes (e.g., add class="ExtraPadding" attribute to the <p> tag). If you look at the answer below this one, you'll also see there is another workaround involving editing the CKEditor configuration to put your custom style back in.
