Additional CSS custom properties for tab and tab-bar

[johnthad]

I'm submitting a:

• bug report
• feature request

What OS are you using?
macOS 10.14.x

What browser(s) is this bug affecting:
all

Current behavior:
From https://unpkg.com/@material/[email protected]/mwc-tab-bar-css.js it appears that the only custom properties for tabs are —mdc-tab-height and --mdc-tab-stacked-height

Expected behavior:
I would like to see more options for customizing tabs in my app. Granted, these ideas may not be comply with material design, but they are items that interest me or folks here:

--mdc-tab-inactive-background-color for tabs not at activeIndex (Currently at each click I'm toggling a class that sets background-color)
--mdc-tab-font-variant and possibly other font-* ("Can't you make that small-caps?")
--mdc-tab-button-radius for the upper corners of the tab buttons
.etc.

[aomarks]

We also have a request for adding a horizontal padding CSS custom property.

[e111077]

also unselected text color

[e111077]

For font customization we will have to defer that to Polymer/lit-element#835