Add header to xaringan-themer.css file

inst/resources/template.css

+/* -------------------------------------------------------
+ *
+ *     !! This file was generated by xaringanthemer !!
+ *
+ *  Changes made to this file directly will be overwritten
+ *  if you used xaringanthemer in your xaringan slides Rmd
+ *
+ *  Issues or likes?
+ *    - https://github.com/gadenbuie/xaringanthemer
+ *    - https://www.garrickadenbuie.com
+ *
+ *  Need help? Try:
+ *    - vignette(package = "xaringanthemer")
+ *    - ?xaringanthemer::write_xaringan_theme
+ *    - xaringan wiki: https://github.com/yihui/xaringan/wiki
+ *    - remarkjs wiki: https://github.com/gnab/remark/wiki
+ *
+ * ------------------------------------------------------- */
 @import url({{text_font_url}});
 @import url({{header_font_url}});
 @import url({{code_font_url}});

vignettes/readme-vignette.Rmd → vignettes/xaringanthemer.Rmd

 ---
 title: "Xaringan CSS Theme Generator"
 author: "Garrick Aden-Buie"
-date: "`r Sys.Date()`"
+date: "2018-05-09"
 output: rmarkdown::html_vignette
 vignette: >
   %\VignetteIndexEntry{Overview of xaringanthemer}