I've given up now because it takes way too long to rebuild and test an iso just to change the default windows theme*. But, does anyone have any ideas why when I load ThemeA into nLite and apply them as the default theme, it applies but ThemeB reverts to classic and I have to select the theme again under desktop properties after install. Maybe I'm doing something wrong with the .theme file because unmodified it works but I don't want sounds, cursors and icons so I've removed those lines. ; Copyright © Microsoft Corp. 1995-2001 [Theme] DisplayName=Theme [Control Panel\Desktop] Wallpaper=(none) TileWallpaper=0 WallpaperStyle=2 Pattern= ScreenSaveActive=0 [VisualStyles] Path=%ResourceDir%\Themes\ThemeName\ThemeName.msstyles ColorStyle=normalcolor Size=NormalSize [MasterThemeSelector] MTSM=DABJDKT ThemeColorBPP=8 Edit: Fixed already I used this as a reference but I thought the icons were also option like the rest but it is enough to make the theme invalid. Hopefully somebody else finds this useful. So the fix is to add the default icon set under [Theme] in the .theme file. ; My Computer [CLSID\{20D04FE0-3AEA-1069-A2D8-08002B30309D}\DefaultIcon] DefaultValue=%WinDir%explorer.exe,0 ; My Documents [CLSID\{450D8FBA-AD25-11D0-98A8-0800361B1103}\DefaultIcon] DefaultValue=%WinDir%SYSTEM32\mydocs.dll,0 ; My Network Places [CLSID\{208D2C60-3AEA-1069-A2D7-08002B30309D}\DefaultIcon] DefaultValue=%WinDir%SYSTEM32\shell32.dll,17 ; Recycle Bin [CLSID\{645FF040-5081-101B-9F08-00AA002F954E}\DefaultIcon] full=%WinDir%SYSTEM32\shell32.dll,32 empty=%WinDir%SYSTEM32\shell32.dll,31 *Once I knew I only had to add a few lines to the .theme file, it's possible to quickly modify the xp disc. Search for the theme, its extension will be .TH_ so rename it to cab and extract the file, modify and then use cmd and makecab the modified theme to create a .TH_ cabinet file again. Then you can replace the old .TH_ file with your fixed one on the disc.