What is your target? you can use CSS and Bootstrap files for styling your app by pointing them from ...\theme\styles\web\css\main.css
is this answering your question? if no please elaborate more.
Short answer: no you can’t. Mendix does not support custom HTML files. All HTML is generated by the MX engine using components and widgets.
The ‘low code’ also extends to the HTML, so no quick template building from Figma, Sketch, XD or whatever: it simply does not work like that.
Sorry mate, no cutting corners here ;)
Also please note that the main.css is not the place to put your custom css. The styles in Mendix are SASS driven and need to be compiled. On every compile the main.css will be overwritten, and your code will be lost.
You can find more about customising your Mendix app with SASS here: https://docs.mendix.com/howto/front-end/calypso
Oh...you don’t have to write SASS, vanilla CSS also works fine: but you still need to compile it. If you are new to this I would advise you to use the Mendix Calypso compiler for that. It’s a nice and solid tool.