The IpHtmlPanel has properties DefaultTypeFace and DefaultFontSize for the standard font. However, these are only used when no other font information is available in the html document or css file.
You could edit the css file of the Lazarus/fpc help system and you could rebuild to help files... Not good, I know...
Simpler, since you seem to be on Windows: Just use the built-in chm viewer of Windows to display the help files. Follow the instructions on
https://wiki.freepascal.org/Installing_Help_in_the_IDE#Installing_CHM_help_.28Lazarus_1.0_and_later.29:
If you are on Windows you can also use the Windows-internal help viewer:
* "HelpExe" should be hh.exe
* "HelpExeParams" must be "%s::%s" (WITH QUOTES, note the double colon).
On Linux you could install one of the chm viewers available and try to set up Lazarus help for it similarly. I did not try it myself, but I am rather sure that it should work, too. (If you find a solution for this idea report it so that the wiki can be updated).