Update to Windows 2000 theme

[joelday]

Let's be reasonable.

It has softer bevels and a horizontal gradient in title bars.

[chungy]

You might be interested in my fork, that I've submitted as PR #10: https://github.com/chungy/CssGitHubWindows/tree/build_themes/Generated_Themes

at least unless and if it gets merged upstream :-)