Since this forum uses the Material Design framework, I think it would be nice if it is used on the homepage, client area, etc. to ensure it matches the forum.
The matter is not what the ‘bones’ or scripts and frameworks behind a service and it’s website is, but how they are connected together to serve the functionality and design of the service. Keep in mind that InfinityFree uses wide range of different CMS, softwares, scripts, custom-made scripts and other programs that ensure the functionality that you, the users can use to host your websites for free.
I am not a licensed designer nor I know much about things like these, but I think that forum’s design matches quite well with main website design and no huge changes need to be done.
The forum is a design built specifically for Vanilla Forums whereas the home page and client area use a different design which was integrated into that code base. Vanilla Forums uses a rather specific structure for it’s layouts, so moving a generic HTML page to Vanilla and vice versa is quite complicated.
Having the same design for the main website and the forum would be ideal, but it’s too much work with too small a reward to make it a priority.