Windows Classic

Windows Classic refers to the built-in native look and feel of Microsoft Windows, specifically the variation that has been introduced with Windows 95 and further refined in later system verisons. The most recent iteration of the classic theme has been introduced in Windows 2000 and Windows ME. Unlike other visual styles, which are implemented using  resource files, the classic theme is implemented directly in the Windows core and offers more customizability of various element sizes, colors and fonts. At its peak, Windows 2000 offered 22 different presets for the classic theme, including 4 high contrast accessibility presets and some of which were also available in larger sizes.

Since ever Windows XP introduced a theme engine, which allowed more complex modification to the system's look and feel, the classic theme is no longer used by default in client versions, but it could still be enabled in the Control Panel. Although server versions also included the theme engine, the classic theme was still used as the default theme due to performance reasons up to Windows Server 2008 R2. Until Windows 8, the classic theme was used to implement high contrast settings. With the introduction of software rendering in the Desktop Window Manager and the Aero Lite visual style, which also allows modification to a degree similar to the classic theme, it is no longer accessible by the user in Windows 8 and equivalent versions, with Windows Server 2012 switching to Aero Lite and Windows PE using Windows Basic, however, the implementation is still present for compatibility reasons and an application can ask the theme engine to disable theming for its windows, which will force the classic theme.

Variations
Starting with Windows 95 and continuing to at least Windows Vista build 5112, and maybe even later, Microsoft included preset variations of the Windows Classic theme that the user could chose from in addition to being able to customize the theme themselves. The styles included such themes as Brick, Desert, and Eggplant among others. Sometime during Windows Vista's development, most likely when the Luna inspired Aero Glass theme was updated to the one seen in the RTM (the start orb added, etc), the preset themes were removed, however, if one has a copy of Windows with the preset themes, you can remake them by editing the Classic theme in Vista/7, and copying the color values over. You can also download the preset themes as well, however, the pack is only compatible with Windows 7.