Inconsistent styles inheritance in NcAvatar

[Antreesy]

Here are some research results:

• Avatar comes as image from server. Its color is defined by method avatarUrlGenerator() in NcAvatar.vue
  Related PR: https://github.com/nextcloud/nextcloud-vue/pull/3119/files
• method avatarUrlGenerator() is called with method loadAvatarUrl(), which is called in mounted hook, but returns different result from the second attempt
• parameter --background-invert-if-dark: invert(100%) is defined by app theme (dark.css) and remains the same in process

So the conclusion is, result of window.getComputedStyle() call is inconsistent due to different browsers and security reasons
Changing the attribute from this.$el to document.body in this place works, because body and its style aren't changing, unlike the unmounted and mounted again avatar.

Originally posted by @Antreesy in nextcloud/spreed#8625 (comment)