To remove the "Create new account" function permanently, using Twig or render hooks is not enough: Drupal-savvy visitors may type the correct URL directly in their browsers. There are bots that attempt to create accounts automatically.
To remove the "Create new account" link AND disable account creation, go to Administration » Configuration » People and ...
The sub theme is fine. Sometimes when you activate a sub theme, you may need to recreate some or all of the block layout. In this case, the primary menu got placed in the header, where it gets a different layout.
Just make note of the block layout in the Bartik theme and duplicate it in the sub theme.
I suspect that your .theme file was not actually empty. Please try the experiment again and check the file size. If it is 3, then your file starts with a Unicode byte order mark (BOM).
The BOM is typically invisible in editors, but it gets sent to the browser before Drupal internally calls the header() function. This causes the error message you have seen.
I have agree with @4k4 you can fix this with database
If your module have been deleted from the system use this query
delete from `key_value` where collection = 'system.schema' and name = '$extension_name';
delete from `key_value` where collection = 'system.schema' and name = 'config_tools';
delete from `key_value` where collection = 'system....
There is (as often) more than one way of solving this.
The dirty and easy way of doing it is with Blocks and Regions, like in Drupal 7. You'll need regions in your theme to be laid out the way you want (and be responsive). Then you manually add nodes/blocks to the regions you have laid out. Even the method of choosing nodes/blocks can be done in different ...