How do I theme the user profile? I would like to create a custom template for the user profile view. With the help of "Theme debug" I figured out that I need to create a template named html--user.html.twig, but even if I copy all content of page.html.twig to my new template, I do get a blank page for any user profile now. Can anyone tell me what I'm doing wrong? I cleared all caches though.

Which content do I have to copy to a custom template (suggested file name)?
html--user.html.twig would be the template file used instead of html.html.twig when viewing a user profile (for example, for http://example.com/user/1). It is not the template file that shows the content of a user profile, but it's the template file that contains the basic structure of a Drupal page; in fact, its default content is the following one.

&lt;!DOCTYPE html&gt;
&lt;html{{ html_attributes }}&gt;
  &lt;head&gt;
    &lt;head-placeholder token="{{ placeholder_token|raw }}"&gt;
    &lt;title&gt;{{ head_title|safe_join(' | ') }}&lt;/title&gt;
    &lt;css-placeholder token="{{ placeholder_token|raw }}"&gt;
    &lt;js-placeholder token="{{ placeholder_token|raw }}"&gt;
  &lt;/head&gt;
  &lt;body{{ attributes }}&gt;
    &lt;a href="#main-content" class="visually-hidden focusable"&gt;
      {{ 'Skip to main content'|t }}
    &lt;/a&gt;
    {{ page_top }}
    {{ page }}
    {{ page_bottom }}
    &lt;js-bottom-placeholder token="{{ placeholder_token|raw }}"&gt;
  &lt;/body&gt;
&lt;/html&gt;


page.html.twig is the template file for a generic page; that is why you are getting a white page for user profile pages.

The template file for a user profile is user.html.twig, and its default content is the following one.

&lt;article{{ attributes }}&gt;
  {% if content %}
    {{- content -}}
  {% endif %}
&lt;/article&gt;


As you see the variable it receives are completely different from the ones the other two template files (html.html.twig and page.html.twig) receive. That is why using the wrong template gives you a blank page.