feat: Highlight text when a poll is multiple choice

[justdaj]

Summary

Recently the words "Multiple Choices" has been added to a poll, but it would be improved further if it was more visually obvious. I appreciate you may not want to deviate too much from upstream, but what about adding a class to the message which can then be styled?

Example:

Quick change to packages/frontend/src/components/MkPoll.vue, line 21 to add a class to the span.

An addition to the CSS to reverse the colours, making background-color: var(--fg) and color: var(--bg); for the new span

Purpose

To make a multiple-choice poll or obvious