rem instead of px units used

[hadasmaimon]

• I have searched the issues of this repository and believe that this is not a duplicate.

Summary 💡

use rem units instead of px units

Examples 🌈

autocomplete
the padding was in pixels and I overrode it

Motivation 🔦

it leaves an opportunity for this change to finally be done to conform with ideal application of font-sizes, padding, & spacing that Web platform enables better than what is enabled by pixels.

The same issue in materiel design:
material-components/material-components-web#2731

[oliviertassinari]

@hadasmaimon Thanks for the issue, I'm closing for #21030, if you want to add anything to the discussion, feel free :).