Update README.md

README.md

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 ![border sample](./samples/border.png)
 
-- `cobalt`, which is heavily based on [elitistsnob/typora-cobalt-theme](https://github.com/elitistsnob/typora-cobalt-theme) and the Cobalt2 theme by [Wes Bos](https://github.com/wesbos/cobalt2/)
+- `cobalt`, which is heavily based on [elitistsnob/typora-cobalt-theme](https://github.com/elitistsnob/typora-cobalt-theme) and the Cobalt2 theme by [Wes Bos](https://github.com/wesbos/cobalt2/).
 
 ![cobalt](samples/cobalt.png)
 
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 ### License
 
 The software is licensed under the [MIT License](https://github.com/rnd195/my-marp-themes/blob/main/LICENSE).
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+### Attribution
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+The `beamer`, `border`, `cobalt`, and `gradient` themes are based on the `default` Marp themes made by [yhatt](https://github.com/marp-team/marp-core/tree/main/themes). The `border` and `gradient` themes import the [Inter](https://fonts.google.com/specimen/Inter) font by [Rasmus Andersson](https://rsms.me/). The `cobalt` theme is based on the Cobalt Typora theme made by [elitistsnob](https://github.com/elitistsnob/typora-cobalt-theme), which was inspired by the Cobalt theme family made by [Wes Bos](https://github.com/wesbos).