Implement dark theme

Closes #4 (closed)

Run preview:

VP_DOCS=true yarn vite build
yarn docs:build
yarn docs:serve

docs/components/button/basic.md

@@ -93,12 +93,12 @@ This is the default style. If you don't specify a style, a filled button is rend
 Outline buttons have a transparent background. Use these to deemphasize the action the button performs.
 
 ```vue-html
-<fw-button outline>
+<fw-button outline color="secondary">
   Outline button
 </fw-button>
 ```
 
-<fw-button outline>
+<fw-button outline color="secondary">
   Outline button
 </fw-button>