Web app structure with python as backend

[scidam]

As I'm more interested in backend I wanted to use free templates for fronted.
1. Is there any source where I can find such free templates [HTML+CSS+JS]?
I would be perfect if there can be user logging feature.

Are you talking about "login feature"?; There are a lot of free html/css/js templates, just search via google.
And you can find templates that include some "registration" fields, which the user should fill and send
his or her data to your server. However, you need to implement what will be happening behind the scene, when someone send the registration data. This could be done by means of Flask, Django etc.

2. Web Server
Can I create such server using python?
I was using Flask so far - as I understand Flask is Web Server it self?

I wouldn't say that. Flask is a framework for building web-applications; Once a user makes a request, and you need
to process this request somehow, there are a lot of micro-tasks raised: e.g. parsing url parameters, requests routing, session management etc. This is where Flask (or other tool) can be helpful.

When we're talking about implementing your own full-featured server, we are talking about quite complex problem; however, if your server is expected to response just "Hello World" (or return a couple of static web-pages),
you can likely implement it.

4. DataBase
Can I build DataBase server also on my own?

It depends. In general, databases are complex things: they solved various problems, such as data consistency,
collision management etc. However, if you database is expected to store only a few textual phrases and you can guarantee that there is no writing collision to it (e.g. users can only read data from it), you can probably build a "database" around "shelve" Python module (or use dict-like structure as an in-memory database).

It is very important in software development (particularly, web developement) to reuse things that
already created and well tested. There are a lot of database servers, web-servers and frameworks; Many of them are open-source. If you want to imagine what amount of work you encounter when trying to implement your own web-server, database-server etc, you could just try to dive into and understand its source code.