default colors in box and tree should be ready for dark mode (#2391)

Author: Fil

src/marks/box.js

@@ -16,7 +16,7 @@ export function boxX(
     x = identity,
     y = null,
     r,
-    fill = "#ccc",
+    fill = "color-mix(in srgb, currentColor 20%, var(--plot-background))",
     fillOpacity,
     stroke = "currentColor",
     strokeOpacity,
@@ -42,7 +42,7 @@ export function boxY(
     y = identity,
     x = null,
     r,
-    fill = "#ccc",
+    fill = "color-mix(in srgb, currentColor 20%, var(--plot-background))",
     fillOpacity,
     stroke = "currentColor",
     strokeOpacity,

src/transforms/tree.js

@@ -65,7 +65,7 @@ export function treeLink({
   path = identity, // the delimited path
   delimiter, // how the path is separated
   curve = "bump-x",
-  stroke = "#555",
+  stroke = "color-mix(in srgb, currentColor 33%, var(--plot-background))",
   strokeWidth = 1.5,
   strokeOpacity = 0.5,
   treeLayout = tree,