Might be pepcak, but for:
<Slide>
<Heading size={2}>
The web is <em>massively</em> moving to the <em>frontend</em>
</Heading>
</Slide>
I get:

However, if I manually specify a bgColor like:
<Slide bgColor="primary">
<Heading size={2}>
The web is <em>massively</em> moving to the <em>frontend</em>
</Heading>
</Slide>
it does pick up the correct background.
/cc @ken_wheeler @paulathevalley