Marketo has a unique syntax that the landing page needs to be coded in in order to be used with the Marketo editors.
There is some documentation in the Product Docs that can help you code: Create a Guided Landing Page Template - Marketo Docs - Product Docs
If you don't want to code, message me and I can provide some alternatives to help you out.
I believe external CSS and JS files work when you publish the landing page but they don't work within Marketo making your page looking a bit :-( when editing your landing page.
You need to put the CSS inline within your HTML file to make it look normal.
Unless anyone gives us a good tip in that regard.