The best way to format an HTML index is through CSS styles. Use the html.stylesheet parameter to specify the name of an external CSS stylesheet, and then create CSS selectors in it to format your index. Here are the selectors you can use.

Main index title:
div.index div.titlepage h2.title

Letter division titles:
div.indexdiv h3

Index term:
div.indexdiv dt

Reference links:
div.indexdiv dt a

If you want to customize how See and See also index entries format, you will need to customize the templates <xsl:template match="indexterm" mode="index-see"> or <xsl:template match="indexterm" mode="index-seealso">, respectively. Those templates are found in html/autoidx.xsl in the stylesheet distribution.