I think that panel is actually the "self.m_normalPlayerTabCtrl" panel, since that has 3 tabbs (units, buildings, adn technologies).
The panel would be set up in the "self.m_normalPlayerTabCtrl = getWBToolNormalPlayerTabCtrl()" call. Unfortunately this is not defined in Python anywhere. It is apparently done by the .exe witht he function being defined via the "CvPythonExtensions" import thing.
However, all the panels are defined via the theme. There is no WB specific stuff in the theme files, so figuring out which one it is using could be a tad time consuming. The basic panels and controls are mostly defined in Resource\Theme\Civ4\Civ4Theme_Custom.thm. The art that is used to show the panels and controls is in Resource\Civ4 (and sub-folders). Since Master of Mana already includes some interface adjustments, it has a full set of Resource files in the mod already.
Changing one panel theme, or the tab control theme, will naturally affect every panel/tab that uses the theme.