[webui] Fix color theme icon layout issue

The size of a cr-theme-icon element is computed incorrectly if the
element doesn't have `display: block` property. This leads to a subtle
bug when displaying a `box-shadow` around a focused element.

Fixed: 1161116
Change-Id: I908b5e997c01db0db00574319c49029ef438b3ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2611025
Auto-Submit: Alex Ilin <alexilin@chromium.org>
Commit-Queue: Esmael Elmoslimany <aee@chromium.org>
Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840651}

ui/webui/resources/cr_components/customize_themes/theme_icon.html

 <style>
   :host {
     --cr-theme-icon-size: 72px;
+    display: block;
   }
 
   :host,