[ecesena]

Other than AWS as in other comments, another solution can be Firebase. You get user authn + hosting, most likely for free.

For theme I've been happy with Landkit (https://landkit.goodthemes.co). It's a bit heavy but you can shrink it.

If you're open to hosted solutions, we're building Saasform. In short, you get landing page + user authn + payments ready to go. Still in early alpha, but if you want to chat more feel free to reach out via email.