Base theme hooks are executed in the incorrect order

Refer to comment #29 in the issue: "Incorrect order and duplicate theme hook suggestions"

I have noticed this same issue, to reiterate the comment above, lets say I have the following theme dependencies:

- foo
-- bar
--- baz

Where baz is the currently enabled theme, which extends bar, which extends foo.

If we were to use hook_theme_suggestions_alter() as an example, Drupal is currently calling these hook implementations in the following order:

• bar_theme_suggestions_alter()
• foo_theme_suggestions_alter()
• baz_theme_suggestions_alter()

This essentially means that bar extends foo, yet foo's hooks override bar.

This isn't the intended order is it?

I beleive this issue stems from \Drupal\Core\Theme\ThemeInitialization::getActiveThemeByName() where it gathers the $base_themes in ascending (reverse) order.

This list of base themes is then used in \Drupal\Core\Theme\ThemeManager::alterForTheme() which takes the list of base themes in reverse order, then tacks on the current theme.

One potential solution for this could be modify \Drupal\Core\Theme\ThemeManager::alterForTheme() to change:

    $theme_keys = [];
    foreach ($theme->getBaseThemes() as $base) {
      $theme_keys[] = $base->getName();
    }

to:

    $theme_keys = [];
    foreach ($theme->getBaseThemes() as $base) {
      $theme_keys[] = $base->getName();
    }
    $theme_keys = array_reverse($theme_keys);