How can I make my own Confluence instance look like this? The default theme uses 100% screen width, and this is better.

[Jeremy Bickerstaffe]

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[Nic Brough [Adaptavist]]

Not sure what the "this" refers to (and full width pages tend to work better for most people, but that's just an opinion).

But to change the overall layout of Confluence, you need to change the "theme" that it uses to render everything.  See https://confluence.atlassian.com/confcloud/applying-a-theme-to-a-site-724765504.html and then dig through the marketplace. There's loads in there, ranging from simple "new theme" through theme managers (like Refined Wiki and Brikit) all the way up to "completely build your own theme from scratch with Themebuilder"