Can I somehow edit libraries.yml files from the backend using the admin panel?
To directly answer your question, yes, you can edit config with the devel module. Install it, then configure it to display the Config Editor.

To answer your question pragmatically, I need to point out that making direct edits to configuration on a regular basis is a recipe for disaster. I think you would be better served using the Asset Injector module, which will allow you to add CSS and JS on the fly, without tinkering with configuration files.

If it is an absolute necessity to modify the library definition, then I think that hook_library_info_alter() is what you need. However, as leymannx points out, libraries aren't intended to be constantly manipulated on the fly. This brings me back to using the Asset Injector. The CSS injector portion is pretty much perfectly suited to your use case; it allows for adding and editing CSS through the admin interface. The CSS injector allows for multiple rules, and these can each be targeted based on path and/or theme.

If you combine this with custom classes attached to the page, then you can use those classes in your CSS rules. You can implement hook_preprocess_page() to do this:

myproject_preprocess_page(&amp;$variables) {
  $node = $variables['node'];
  if ($node) {
    // Assume that the Taxonomy Vocabulary machine name is "category".
    /** @var \Drupal\taxonomy\Entity\Term[] $terms */
    $terms = \Drupal::entityTypeManager()
      -&gt;getStorage("taxonomy_term")
      -&gt;loadByProperties(["vid" =&gt; "category"]);
    // Get all categories present on the current node.
    $categories = $node-&gt;get('field_category')-&gt;getValue();
    // Get just the TIDs as a simplified array.
    $categories = array_column($categories, 'target_id');
    // Ensure that there is a class array.
    $variables['attributes']['class'] = $variables['attributes']['class'] ?? [];
    foreach($categories as $category) {
      // Craft a HTML-compliant class name based on the label.
      $variables['attributes']['class'][] = \Drupal\Component\Utility\Html::cleanCssIdentifier($terms[$category]-&gt;label());
    }
  }
}