Funny that thing
A bit blurry for my eyes, but interesting to play with...
Well, I'm on KM 1.6beta and it works fine here, but obviously not yet in 1.5.4. You can google for "text-shadow css" for more information, and here's a bit info and examples for different shadow values:
supported in Safari since version 3 (also available for Windows), Opera since 9.5, Firefox since 3.1, Google Chrome since version 2, Konqueror and iCab. In fact, text-shadow is supported by all browsers that are based on WebKit, the rendering engine behind Safari and Chrome. Internet Explorer 8 does not support such text shadows (except for some DirectX image transform filters).
FF 3.1 means no chance for KM1.5.4, you need the gecko engine that's used in KM1.6 or newer.