Add preview images to the themes menu (#3444)

* Add new script for themes menu

* remove icons from the themes menu

* Make theme titles human readable and add spacing at the end

* Migrate to using a link for the new menu generator

* Retain consistent width

But we still do have movement when filtering the menu down to a
shorter-than-longest name. Should try to stabilize that.

* Update all previews to 2K resolution and same dimensions

* Update script to use first bg file if theme preview file doesn't exist

* Add padding to preview box instead of spaces to theme name

* Consistent formatting/indenting

* Prevent height jump

* Revert padding change and remove background color

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Co-authored-by: David Heinemeier Hansson <david@hey.com>

default/walker/themes/omarchy-default/style.css

@@ -113,5 +113,4 @@ child:selected .item-box * {
 }
 
 .preview {
-  background: @background;
 }