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This plugin adds Theme menus in the admin bar. 
You can switch and activate the theme on the admin bar. 
You don’t need to go to the administration screens to switch the theme.
(For Japanese User) 管理バーにテーマメニューを追加します。 管理バー上でサイトテーマの変更・有効化が行えます。 テーマを変更するために管理画面に行く? 要がなくなります。

Switch theme on admin bar

This plugin adds Theme menus in the admin bar. You can switch and activate the theme on the admin bar.

Xanthonius

I created this plugin to help when creating sites that have fixed headers, if you fix the header to the top of the screen you then have to go in and add additional styles for logged in users that have the admin bar, after doing this on quite a lot of sites I thought a better way to manage this might be to allow the admin bar to be toggled in and out of view on the front end, whilst still keeping everything fixed to the top of the browser!
The plugin simply adds a button to the far right of the admin bar, shifting the whole bar up past the top of the screen (out of view) except for the button, on click of the button the admin bar slides into view.

Toggleable Admin Bar

Allows you to toggle the admin bar on the front end. Useful for websites with fixed positioned elements where the admin bar is in the way.

Switch theme on admin bar vs Toggleable Admin Bar