[Component amendment]: p-chip is-readonly paper theme

## Component/pattern to amend

The default/gray `p-chip is-readonly` chip is the same color as the `paper` theme background color. I think the readonly chip should: 1) have an ever so subtle outline; 2) be dark theme on `paper`; 3) be a different color?

## Visual

## Context

Websites (such as certification.canonical.com) that use the `paper` theme would like to use readonly chips, but they are not very distinguishable as chips due to the color of the chip and background being the same.

Start by locating the p-chip styles and the paper theme styles in the Vanilla Framework source. Compare the readonly chip against the paper background and choose a treatment that makes it distinguishable, then verify the component in the paper theme and confirm the readonly state remains visually clear.

Written by the indexing model from the issue text.