Custom scrollbars for both firefox and webkit browsers

To consider the following 4 cases (this sample doesn't show any custom style for IE):

• Webkit-based browsers. Scrollbar can be fully styling using ::-webkit-scrollbar, see https://codepen.io/waterplea/pen/dVMopv for example. ~90% of all users (see https://caniuse.com/#search=%3A%3A-webkit-scrollbar).
• Firefox v 64+ (released > 2 years ago). Scrollbar can be styled using scrollbar-width and scrollbar-color properties. For instance, this example works in FF: https://codepen.io/team/css-tricks/pen/QWWXQzE. ~4% of all users (see https://caniuse.com/#search=scrollbar-width).
• IE. Scrollbar can be somehow styled using special properties like scrollbar-base-color. ~1.6% of all users (see https://caniuse.com/#search=scrollbar-base-color).
• Other browsers/users. <5%

See the Pen custom scrollbar (firefox and webkit browsers) by Rafa (@reacraf) on CodePen.

Please note: for vertical scroll bars, you need to set a width size prop under WebKit prefixed rules.