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Mastodon instances have a variety of different themes to choose from.
You can change your Mastodon theme by going to Preferences, then Appearance, then Site Theme, then click on "Save Changes".
This is particularly useful if you have several different accounts open, so you can switch between them without getting them mixed up.
Different instances have different selections of themes, ask your admin if you want extra themes added.
@feditips And there's also a difference between the one column(simple layout) or the multi column(advanced layout)! 
@stux Yup, saving that for a future post 
@feditips Nicee! 
Your making great content, thank you!