Added custom theme tutorial docs

website/docs/getting-started/custom-theme.md

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+---
+id: custom-theme
+title: Custom theme
+sidebar_position: 4
+---
+
+We've made hundreds of custom components across dozens of projects, including complete custom themes. Most of them began as source forks of `uniforms-unstyled` - one simply copies the source and imports `./some/project/path/uniforms-custom-theme-with-a-cool-name`.
+
+For the purposes of this tutorial we will be using `uniforms-custom-theme` as our custom theme name.
+
+## Copy theme source
+
+Copy source of `uniforms-unstyled` package from uniforms repository, or any other provided theme that you would like to extend, and put it somewhere inside your project, e.g. `./uniforms-custom-theme`.
+
+Now you can start making changes to your own custom theme!
+
+## Import newly created theme
+
+Instead of importing themes from `node_modules` (original uniforms theme packages), simply import theme from your local path.
+
+That's all!
+
+## Referenced issues and pull requests
+
+[#433](https://github.com/vazco/uniforms/issues/433) [#609](https://github.com/vazco/uniforms/issues/609#issuecomment-545079686) [#612](https://github.com/vazco/uniforms/pull/612#issuecomment-545643935)