You need to overwrite the header and footer templates from the modules (or parent theme if you're using one). You can find these in
So you will need to create
And add your markup in there, but please note it isn't a case of copying and pasting. You will need to convert your markup into Magento friendly markup, such as using the PHP translations and adding static blocks if it needs to be managed through the admin.
To extend your parent theme styling you will need to create this file:
And add any of your JS into
app/design/frontend/PartyShowroom/default/Magento_Theme/web/js and load it via Require JS.
This is the basics to adding your content to a Magento theme, there is too much for me to give a full answer on how to convert your theme. The dev docs are your friend for this.