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Open Krunker.io in your browser.
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Open Developer Tools (F12 or right-click → Inspect).
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Go to the Sources tab → Top → krunker.io
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Navigate to the css folder → open main_custom.css.
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Paste your custom CSS code into this file.
If you see a paste warning, type allow pasting in the console or prompt to proceed.
If the new theme is not applied reload the game page to see your new css applied.