The theme for FiM++ Wiki is heavily influenced by Metro UI. If what you want to do is not listed, look to Microsoft.
Colors
Only use the colors described in FiM++ Wiki:Colors and only for the purposes they are given, there. Any other colors have to be approved by the head admin.
Borders
Borders are discouraged. They should only be used if no other option presents itself.
Columns
Sometimes there is a lot of small pieces of content in a section (such as a list of phrases), and we don't want that to mean that the page becomes unnecessarily tall. In order to avoid this, you should put the items into columnar lists. BUT WAIT!!! Don't actually separate your list into several pieces; this makes it difficult to edit and not dynamic. Instead, use the CSS class "autoCol". Let's say you have this:
* Long list item 1 * Long list item 2 ... * Long list item 182 * Long list item 183
to make this automatically wrap into several columns, do this:
<DIV CLASS="autoCol"> * Long list item 1 * Long list item 2 ... * Long list item 182 * Long list item 183 </DIV>
The "autoCol" class ensures that the content is put into several evenly-spaced columns if and only if the user's browser supports the CSS columns property (to check which browsers support this, see w3schools' page on said property). The exact number of columns will change depending on the width of the viewing window. This is not supported in the Wikia default skin because its pages are always the same width.
Example
A good live example is the List of Reserved Phrases page.