[customize themes] Fix layouting of third-party theme box

This CL makes the third-party theme box appear above the Chrome colors
instead of beside them.

Bug: 1140128
Change-Id: Ie8f0e03230f23ef49bc71e7c60866160bdc1ebed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2488392
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: dpapad <dpapad@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Auto-Submit: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819500}

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

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