chaging background of readme md code example

Example 1: how to add color in text github readme

```diff
- text in red
+ text in green
! text in orange
# text in gray
@@ text in purple (and bold)@@
```

Example 2: change text color in md file

<div class="text-white bg-blue mb-2">
  .text-white on .bg-blue
</div>
<div class="bg-blue-light mb-2">
  .text-gray-dark on .bg-blue-light
</div>
<div class="text-white bg-red mb-2">
  .text-white on .bg-red
</div>
<div class="text-red bg-red-light mb-2">
  .text-red on .bg-red-light
</div>
<div class="bg-green-light mb-2">
  .text-gray-dark on .bg-green-light
</div>
<div class="bg-yellow mb-2">
  .text-gray-dark on .bg-yellow
</div>
<div class="bg-yellow-light mb-2">
  .text-gray-dark on .bg-yellow-light
</div>
<div class="text-white bg-purple mb-2">
  .text-white on .bg-purple
</div>
<div class="text-white bg-gray-dark mb-2">
  .text-white on .bg-gray-dark
</div>
<div class="bg-gray">
  .text-gray-dark on .bg-gray
</div>