Via Community Store (recommended)
To install this theme via the community store, navigate to  Settings > Appearance
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Under Themes, click Manage
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Type "Flexcyon in the search bar > Select it > Click "Install and Use"
Via BRAT
To install this theme via BRAT, navigate to Settings > Community Plugins > Browse
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Type "BRAT" in the search bar > Select it (the one by TftHacker) > Click install
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Wait for installation to complete then click Enable
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Click Options
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Click Add Beta Theme
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Input https://github.com/bladeacer/flexcyonand then clickAdd Theme
Style Settings Plugin
Installation of Style Settings is highly recommended for this theme, as most customisation and functionality is built around it.
Documentation for style settings of this theme can be found here