fix arcCollatzUp snapshot for dark mode (use currentColor) (#2368)

Author: Fil

test/output/arcCollatzUp.svg

@@ -52,12 +52,12 @@ export async function arcCollatzUp() {
       () =>
         svg`<defs>
         <linearGradient id="gradient0" x1="0%" y1="0%" x2="100%" y2="0%">
-          <stop offset="5%" stop-opacity="0.3"></stop>
-          <stop offset="95%" stop-opacity="1"></stop>
+          <stop offset="5%" stop-color="currentColor" stop-opacity="0.3"></stop>
+          <stop offset="95%" stop-color="currentColor" stop-opacity="1"></stop>
         </linearGradient>
         <linearGradient id="gradient1" x1="0%" y1="0%" x2="100%" y2="0%">
-          <stop offset="5%" stop-opacity="1"></stop>
-          <stop offset="95%" stop-opacity="0.3"></stop>
+          <stop offset="5%" stop-color="currentColor" stop-opacity="1"></stop>
+          <stop offset="95%" stop-color="currentColor" stop-opacity="0.3"></stop>
         </linearGradient>`
     ]
   });