How (or Where) do I define my own style of drop-down box?

(i) the size, styles and fonts in the box?

(ii) What if I want to give multiple categories of suggestions, coming from different sources of data?
For example, I want to provide list of (a) words beginning with same characters in the upper part (b) synonims in the lower part within the same box.

(iii) Do I have to code in separate Javascript?
Or can I just code in VB.NET or C# together with AJAX controls, like AutoCompleteExtender?

Your tips will be much appreciated!!

hk