add theme to styled-components example

[Tomekmularczyk]

Seems that to some it is still unclear how to add theme when working with styled-components.
I thought it would be good to have it set up in the example.

[github-actions]

Stats from current PR

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	zeit/next.js canary	Tomekmularczyk/next.js add-theme-to-styled-components-example	Change
Build Duration	13.2s	12.7s	-474ms
node_modules Size	39.6 MB	39.6 MB	⚠️ +40 B
Total Bundle (main, webpack, commons) Size	207 kB	207 kB	✓
Total Bundle (main, webpack, commons) gzip Size	68.2 kB	68.2 kB	⚠️ +1 B
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Client _app gzip Size	919 B	919 B	✓
Client _error Size	14.2 kB	14.2 kB	✓
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Client pages/index gzip Size	222 B	222 B	✓
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Client pages/link gzip Size	1.63 kB	1.63 kB	✓
Client pages/routerDirect Size	411 B	411 B	✓
Client pages/routerDirect gzip Size	296 B	296 B	✓
Client pages/withRouter Size	393 B	393 B	✓
Client pages/withRouter gzip Size	280 B	280 B	✓
Client main Size	27.9 kB	27.9 kB	✓
Client main gzip Size	9.64 kB	9.64 kB	⚠️ +1 B
Client commons Size	177 kB	177 kB	✓
Client commons gzip Size	57.4 kB	57.4 kB	✓
Client webpack Size	1.49 kB	1.49 kB	✓
Client webpack gzip Size	769 B	769 B	✓
Base Rendered Size	1.34 kB	1.34 kB	✓
Build Dir Size	638 kB	638 kB	✓

Click to expand serverless stats

	zeit/next.js canary	Tomekmularczyk/next.js add-theme-to-styled-components-example	Change
Build Duration	15.7s	15.5s	-187ms
node_modules Size	39.6 MB	39.6 MB	⚠️ +40 B
Total Bundle (main, webpack, commons) Size	207 kB	207 kB	✓
Total Bundle (main, webpack, commons) gzip Size	68.2 kB	68.2 kB	✓
Client _app Size	2.13 kB	2.13 kB	✓
Client _app gzip Size	919 B	919 B	✓
Client _error Size	14.2 kB	14.2 kB	✓
Client _error gzip Size	5.39 kB	5.39 kB	✓
Client pages/index Size	288 B	288 B	✓
Client pages/index gzip Size	222 B	222 B	✓
Client pages/link Size	3.62 kB	3.62 kB	✓
Client pages/link gzip Size	1.63 kB	1.63 kB	✓
Client pages/routerDirect Size	411 B	411 B	✓
Client pages/routerDirect gzip Size	296 B	296 B	✓
Client pages/withRouter Size	393 B	393 B	✓
Client pages/withRouter gzip Size	280 B	280 B	✓
Client main Size	27.9 kB	27.9 kB	✓
Client main gzip Size	9.64 kB	9.64 kB	✓
Client commons Size	177 kB	177 kB	✓
Client commons gzip Size	57.4 kB	57.4 kB	✓
Client webpack Size	1.49 kB	1.49 kB	✓
Client webpack gzip Size	769 B	769 B	✓
Serverless pages/link Size	327 kB	327 kB	✓
Serverless pages/link gzip Size	87.6 kB	87.6 kB	-2 B
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Serverless pages/_error gzip Size	85.4 kB	85.4 kB	-1 B
Serverless pages/routerDirect Size	322 kB	322 kB	✓
Serverless pages/routerDirect gzip Size	85.7 kB	85.7 kB	-1 B
Serverless pages/withRouter Size	322 kB	322 kB	✓
Serverless pages/withRouter gzip Size	85.8 kB	85.8 kB	-1 B
Build Dir Size	2.19 MB	2.19 MB	✓