Added --plyr-video-background for having control over the background of a video with alpha channel (webm) or a poster image with alpha channel. (#2076)

src/sass/components/poster.scss

@@ -3,7 +3,7 @@
 // --------------------------------------------------------------
 
 .plyr__poster {
-  background-color: #000;
+  background-color: var(--plyr-video-background, $plyr-video-background);
   background-position: 50% 50%;
   background-repeat: no-repeat;
   background-size: contain;

src/sass/settings/colors.scss

@@ -3,6 +3,7 @@
 // ==========================================================================
 
 $plyr-color-main: var(--plyr-color-main, hsl(198, 100%, 50%)) !default;
+$plyr-video-background: var(--plyr-video-background, rgba(0,0,0,1)) !default;
 
 // Grayscale
 $plyr-color-gray-900: hsl(216, 15%, 16%) !default;

src/sass/types/video.scss

@@ -4,7 +4,7 @@
 
 // Container
 .plyr--video {
-  background: #000;
+  background: var(--plyr-video-background, $plyr-video-background);
   overflow: hidden;
 
   &.plyr--menu-open {
@@ -13,7 +13,7 @@
 }
 
 .plyr__video-wrapper {
-  background: #000;
+  background: var(--plyr-video-background, $plyr-video-background);
   height: 100%;
   margin: auto;
   overflow: hidden;