Is it possible to use online server like for example the File Manager in one.com?

It does not matter if you use a VPS, or a hosting service or host it yourself (if you have the right hardware). Its always your decision there is no "NO WAY thats much better"... I prefer using a Linux VPS and host it myself on that. But thats not what casuals should do because most dont have that much experience in Linux (but they could learn something new...)

There is no wrong or right...

Your text look like a ad btw

Hi,

Thank you for quickly answer!

So that means that I could build a Wordpress theme from scratch, by using CSS, HTML and Javascript, in for example the File Manager of the hosting provider, one.com?

Best Regards, Gerhard Øpsen

So that means that I could build a Wordpress theme from scratch, by using CSS, HTML and Javascript, in for example the File Manager of the hosting provider, one.com?

Theoretically yes you could. In practice nobody does that in the development phase. There a bunch of reasons including security, performance, SEO reasons and being altogether a very unpractical workflow. In the development phase you are likely to output debugging data that could be used to exploit your site. As for SEO reasons imagine your site beeing crawled by Google in a unfinished state - that would be a very bad start. Of course you could avoid this by adjusting your robots.txt and adding password protection via .htaccess, but then things tend to become very unpractical.

Best practise is setting up a local environment for WordPress (Mamp, Xamp, Vagrant, or setting up your own Apache Server on MacOS or Linux) and then uploading your changes to production (your hosting server) using SSH or FTP and updating your database by importing your MySQL Dump. That said, when it comes to small fixes and changes It's totally okay to just connect to your server via SSH or FTP and edit files on the fly. You should have backups though in case something goes wrong or use version control like git.