It is a difficult topic and one which isn't obvious to the uninitiated! The worst thing is if you design a website on a fairly dark monitor and miss things, as then other people will see marks, etc on your graphics that you missed!

You can change your 'gamma' but there's no right or wrong - just what you prefer. Luckily all PCs have roughly the same gamma level by default, although Macs have a higher gamma.

The best thing to do is to always ensure that your designs don't have blemishes and always make your text, etc, stand out so that if someone is viewing on a darker screen the difference is still noticeable.