The VVS Theme Viewer plugin exists to assist in the creation of custom themes. Using the plugin, you can:
List all preinitialized themes (builtin ones as well as XML themes in the VVS install directory that were loaded at game start).
Export any preinitialized theme to XML format.
Switch to any preinitialized theme by name.
Load an XML theme temporarily from another location. The purpose of this is to ease tweaking, so a small change can be made and then the theme reloaded without having to restart AC.