Fix formatting of css variables in template variables vignette

vignettes/template-variables.Rmd

 for example `background_color = "#EAEAEA"`.
 
 ```{r table, results = "asis", echo=FALSE}
-template_variables <- xaringanthemer:::template_variables
-template_variables$variable <- glue::glue_data(template_variables, "`{variable}`")
-template_variables$default <- gsub("[{}]", "", template_variables$default)
-template_variables <- template_variables[, c(
+tv <- xaringanthemer:::template_variables
+tv$variable <- glue::glue_data(tv, "`{variable}`")
+tv[!is.na(tv$css_variable), "css_variable"] <- glue::glue("`{tv$css_variable[!is.na(tv$css_variable)]}`")
+tv[is.na(tv$css_variable), "css_variable"] <- ""
+tv$default <- gsub("[{}]", "", tv$default)
+tv <- tv[, c(
   "variable", "description", "element", "default", "css_variable"
 )]
 knitr::kable(
-  template_variables,
+  tv,
   col.names = c("Variable", "Description", "Element", "Default", "CSS Variable")
 )
 ```