Use a consistent background colour for video tiles and avatars (#2868)

src/index.css

@@ -47,6 +47,11 @@ layer(compound);
   --background-gradient: url("graphics/backgroundGradient.svg");
 }
 
+:root,
+[class*="cpd-theme-"] {
+  --video-tile-background: var(--cpd-color-bg-subtle-secondary);
+}
+
 .cpd-theme-dark {
   --cpd-color-border-accent: var(--cpd-color-green-1100);
   --stopgap-color-on-solid-accent: var(--cpd-color-text-primary);

src/room/VideoPreview.module.css

@@ -18,8 +18,7 @@ Please see LICENSE in the repository root for full details.
   width: 100%;
   height: 100%;
   object-fit: cover;
-  background-color: black;
-  background-color: var(--cpd-color-bg-subtle-primary);
+  background-color: var(--video-tile-background);
 }
 
 video.mirror {
@@ -35,7 +34,7 @@ video.mirror {
   display: flex;
   justify-content: center;
   align-items: center;
-  background-color: var(--cpd-color-bg-subtle-secondary);
+  background-color: var(--video-tile-background);
 }
 
 .buttonBar {

src/tile/MediaView.module.css

@@ -15,7 +15,7 @@ Please see LICENSE in the repository root for full details.
   inline-size: 100%;
   block-size: 100%;
   object-fit: contain;
-  background-color: var(--cpd-color-bg-subtle-primary);
+  background-color: var(--video-tile-background);
   /* This transform is a no-op, but it forces Firefox to use a different
   rendering path, one that actually clips the corners of <video> elements into
   the intended rounded shape. We can remove this if Firefox stops being broken. */
@@ -35,7 +35,7 @@ Please see LICENSE in the repository root for full details.
 }
 
 .bg {
-  background-color: var(--cpd-color-bg-subtle-secondary);
+  background-color: var(--video-tile-background);
   inline-size: 100%;
   block-size: 100%;
   border-radius: inherit;