Add theme selection menu

[Andreasgdp]

Short description of a problem or missing feature

Add a section to select the theme for the site. Could be as showed below

Currently it is just a slider

• #90

[create-issue-branch]

Hi there 👋

Using this App for a private organization repository requires a paid subscription that you can buy on the GitHub Marketplace

If you are a non-profit organization or otherwise can not pay for such a plan, contact me by creating an issue

Since you activated this App before the paid plan introduction it keeps on working for this repository. Upgrade to the paid plan to remove this message.

[create-issue-branch]

Branch issue-64-Add_theme_selection_menu created!

[create-issue-branch]

Branch issue-64-Add-theme-selection-menu created!

[Andreasgdp]

Blocked by #90