[vercel-tutor] Fix border/background color for toolbar button (#1088)

### Description

The button was not visible in light mode.

### Demo URL

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### Type of Change

- [ ] New Example
- [ ] Example updates (Bug fixes, new features, etc.)
- [ ] Other (changes to the codebase, but not to examples)

### New Example Checklist

- [ ] 🛫 `npm run new-example` was used to create the example
- [ ] 📚 The template wasn't used but I carefuly read the [Adding a new
example](https://github.com/vercel/examples#adding-a-new-example) steps
and implemented them in the example
- [ ] 📱 Is it responsive? Are mobile and tablets considered?

vercel-tutor/app/smaller-triangle.tsx

@@ -47,7 +47,7 @@ function Check() {
 
 function Toolbar() {
   return (
-    <span className="rounded-full border border-white w-[24px] h-[24px] inline-flex items-center justify-center align-middle">
+    <span className="rounded-full border border-secondary bg-[rgba(0,0,0,.8)] w-[24px] h-[24px] inline-flex items-center justify-center align-middle">
       <svg
         aria-hidden="true"
         width="24"