Update README.md

Fixed broken URL

css/theme/README.md

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 ## Dependencies
 
-Themes are written using Sass to keep things modular and reduce the need for repeated selectors across files. Make sure that you have the reveal.js development environment installed before proceeding: https://revealjs.revealjs.com/installation/#full-setup
+Themes are written using Sass to keep things modular and reduce the need for repeated selectors across files. Make sure that you have the reveal.js development environment installed before proceeding: https://revealjs.com/installation/#full-setup
 
 ## Creating a Theme