[docs] Polishing out the date-picker playground

[zanivan]

• [x] I have followed (at least) the PR section of the contributing guide.

While checking out the X docs, I noticed that the playground borders seemed too bright against the dark theme, which was a bit off. So, I decided to tweak them a bit and ended up making some other adjustments along the way.

Current	New

👉 https://deploy-preview-11869--material-ui-x.netlify.app/x/react-date-pickers/date-picker/#customization

[zanivan]

@noraleonte I just made some stray tweaks, please let me know if anything needs to be fixed, since this is my first PR for X :)

[mui-bot]

Deploy preview: https://deploy-preview-11869--material-ui-x.netlify.app/

Generated by :no_entry_sign: dangerJS against 0930b5e78cf6d8969042044e6113d09dfec3bde3

[noraleonte]

I pushed a small fix to hopefully make the CI happy 🤞

[zanivan]

Just a little bump here @noraleonte :)