When a new help page is created using the helpauthoring tool, then the styles are taken form the template xmlhelptemplate.ott
When an existing help file is edited using the helpauthoring tool, then the styles are set in the transformation xmlhelp2soffice.xsl
When the user later on looks at the help page, then the styles from default.css are used.

And all these use different styles. There should be me more consistency regarding which fonts are used and which size is used. Especially it should be avoided to use non common fonts.
For example for headings you have in the en-US default.css the setting
{ font-family: "Albany AMT",Albany,Arial,"Nimbus Sans L","Bitstream Vera Sans",Helvetica,Lucida,Geneva,Helmet,"Andale Sans UI","Arial Unicode MS","Lucida Sans Unicode",Tahoma,sans-serif; }
In the transformation xmlhelp2soffice.xsl you have the setting style:font-name="Droid Sans" (?!)
For the template "Liberation Serif" or "Liberation Sans" are discussed.
The Wikihelp has its own styles in addition.

For the default.css the bug #82997 exists, but I think, that it should be solved more generally.