github readme add background color section code example

Example 1: 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>

Example 2: change text color in md file

- ![#f03c15](https://via.placeholder.com/15/f03c15/000000?text=+) `#f03c15`
- ![#c5f015](https://via.placeholder.com/15/c5f015/000000?text=+) `#c5f015`
- ![#1589F0](https://via.placeholder.com/15/1589F0/000000?text=+) `#1589F0`