Tell Tailwind to use the Docusorus dark-mode attribute (#1573)

## Description

Tailwind needs to know what is the attribute specifying DarkMode. Default is `class="dark"` which is not what Docusaurus uses. 

## Type of change

<!--- Please check the type of change your PR introduces: --->
- [ ] 🌻 Feature
- [x] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Test
- [ ] 💻 CI/Deployment
- [ ] 🐹 Trivial/Minor

## Issue(s)

<!-- Can reference multiple issues. Use one of the following "magic words" - "closes, fixes" to auto-close the Github issue. -->

## Test Plan

<!-- How will this be tested prior to merging.-->
- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

docs/tailwind.config.js

@@ -1,6 +1,7 @@
 /** @type {import('tailwindcss').Config} */
 module.exports = {
   content: ["./src/**/*.{js,jsx,ts,tsx}"],
+  darkMode: ['class', '[data-theme="dark"]'],
   theme: {
     screens: {
       xs: "540px",