Document new argument in a few more places

NEWS.md

   presentations, and they support a wide variety of languages and weights. 
   Another reason for the change is that the xaringan (remarkjs) default body 
   font, _Droid Serif_, is no longer officially included in Google Fonts.
+  
+* Additional colors can be provided in the `colors` argument, which takes a
+  named vector of colors. The names become both CSS classes and CSS variables
+  that can be used throughout your slide styles. For example,
+  `colors = c(red = "#d34213")` creates a `.red` CSS class for red foreground 
+  text, a `.bg-red` CSS class for a red background, and a `--red` CSS variable
+  that can be referenced as `var(--red)` in any `_color` argument of your style
+  function.
 
 
 ## Bugfixes and Improvements

README.md

 Note that the colors used below are for demonstration only, the point of
 the `style_` functions is for you to choose your own color palette\!
 
+If you color palette uses more than two colors, you can add additional
+colors with the `colors` argument. See the [Colors](#colors) section for
+more information.
+
 ### Monotone
 
 Use these functions to automatically create a consistent color palette

man/fragments/_themes.Rmd

 Note that the colors used below are for demonstration only,
 the point of the `style_` functions is for you to choose your own color palette!
 
+If you color palette uses more than two colors,
+you can add additional colors with the `colors` argument.
+See the [Colors](#colors) section for more information.
+
 ### Monotone
 
 ```{r include=FALSE}

vignettes/themes.Rmd

 
 ```{r child="../man/fragments/_themes.Rmd"}
 ```
+
+
+## Colors
+
+```{r child="../man/fragments/_colors.Rmd"}
+```