Apply "display: inline-flex" to cr-customize-themes

Otherwise, the element has 0px by 0px size which makes it difficult to
style.

Bug: 1129186
Change-Id: I92c32daabccfc56bc0eaf95103215093cf1a2cb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414278
Auto-Submit: Alex Ilin <alexilin@chromium.org>
Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808000}

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

@@ -2,6 +2,7 @@
   :host {
     --cr-customize-themes-grid-gap: 20px;
     --cr-customize-themes-icon-size: 72px;
+    display: inline-flex;
   }
 
   #thirdPartyThemeContainer {