Use black background for sheet padding (#4)

- Use black background for sheet padding (cc1d283e1a6583dc49cdacdf4cdc2b0275d99df2)

app.py

@@ -232,7 +232,7 @@ def _gallery_paths(gallery):
     return paths
 
 
-def _fit_contain(im, cw, ch, bg=(255, 255, 255)):
+def _fit_contain(im, cw, ch, bg=(0, 0, 0)):
     """Scale `im` to fit *fully* inside (cw, ch) keeping aspect ratio, then center it on a
     padded cell. Unlike ImageOps.fit (cover + center-crop), nothing is cropped off."""
     fitted = ImageOps.contain(im, (cw, ch), Image.LANCZOS)
@@ -249,7 +249,7 @@ def compose_sheet(gallery):
     imgs = [Image.open(p).convert("RGB") for p in paths]
     if len(imgs) == 1:
         return imgs[0]
-    BG = (255, 255, 255)
+    BG = (0, 0, 0)
     CW, CH = 1536, 896
     canvas = Image.new("RGB", (CW, CH), BG)
     cols = math.ceil(math.sqrt(len(imgs))); rows = math.ceil(len(imgs) / cols); g = 16