Return default theme when unknown theme value is passed

[envex]

What does this implement/fix?

Previously we threw an error when a theme could not be found, but that would cause the parent component to crash.

Now we'll return the default theme and warn in the console instead of an Error.

Resolves #1659

Storybook link

https://6062ad4a2d14cd0021539c1b-dhhtmrpotq.chromatic.com/?path=/story/polaris-viz-charts-barchart-chromatic--bad-theme

Before merging

• Check your changes on a variety of browsers and devices.

• Update the Changelog's Unreleased section with your changes.

• Update relevant documentation, tests, and Storybook.

• Make sure you're exporting any new shared Components, Types and Utilities from the top level index file of the package

[github-actions]

size-limit report 📦

Path	Size	Loading time (3g)	Running time (snapdragon)	Total time
polaris-viz-core-cjs	61.3 KB (+0.07% 🔺)	1.3 s (+0.07% 🔺)	933 ms (-25.92% 🔽)	2.2 s
polaris-viz-cjs	216.09 KB (+0.02% 🔺)	4.4 s (+0.02% 🔺)	2 s (+6.93% 🔺)	6.3 s
polaris-viz-esm	176.39 KB (+0.02% 🔺)	3.6 s (+0.02% 🔺)	1.1 s (-15.39% 🔽)	4.6 s
polaris-viz-css	4.79 KB (0%)	96 ms (0%)	274 ms (-26.2% 🔽)	370 ms
polaris-viz-esnext	182.35 KB (+0.02% 🔺)	3.7 s (+0.02% 🔺)	1.5 s (-5.62% 🔽)	5.2 s

[michaelnesen]

Nice 🚀