[FIX] lightmode when alpha channel = 0

[This issue][1] reported that the darkmode of the panel could be applied although the background is white.

When the alpha channel is 0, it takes the color of the document which is `rgb(0,0,0,0)` by default. The extension would thus think the background is black.

Now if the alpha channel is 0, it default to light mode.

[1]: #39

src/utils.js

@@ -320,6 +320,9 @@ function getBackgroundColor() {
 function isDarkMode() {
   try {
     const rgba = textColorToRGBA(getBackgroundColor());
+    if (rgba[3] === 0) {
+      return false;
+    }
 
     const av = rgba.slice(0, 3).reduce((a, v) => a + v) / 3;
     return av < 128;