read_css_vars <- function(file = NULL) {
  if (is.null(file)) {
    css_candidates <- find_xaringan_themer_css()
    if (!length(css_candidates)) {
      stop("Unable to locate a xaringanthemer css file.")
    } else if (length(css_candidates) == 1) {
      file <- css_candidates
    } else if (length(css_candidates) > 1) {
      is_xaringan_themer_css <- grepl("xaringan-themer.css", css_candidates, fixed = TRUE)
      if (any(is_xaringan_themer_css)) {
        file <- css_candidates[is_xaringan_themer_css][1]
      } else {
        file <- css_candidates[1]
        message(glue::glue("Using xaringanthemer theme in {file}"))
      }
    }
  }

  css_get_root(file)
}

find_xaringan_themer_css <- function() {
  css_files <- list.files(pattern = "css$", recursive = TRUE, full.names = TRUE)
  css_files_head <- purrr::map(css_files, readLines, n = 5)
  is_xt <- grepl(pattern = "generated by xaringanthemer", css_files_head, fixed = TRUE)

  css_files[is_xt]
}

css_get_root <- function(file) {
  x <- readLines(file, warn = FALSE)
  x <- paste(x, collapse = "\n")
  where <- regexpr(":root\\s*\\{[^}]+\\}", x)
  if (where < 0) return(NULL)
  x <- substr(x, where, where + attr(where, "match.length"))
  x <- strsplit(x, "\n")[[1]]
  m <- regexec("--(.+):\\s*(.+?);", x)
  x <- regmatches(x, m)
  x <- purrr::compact(x)
  vars <- gsub("-", "_", purrr::map_chr(x, `[`, 2))
  values <- purrr::map(x, `[`, 3)
  names(values) <- vars
  for (font_type in c("text", "header", "code")) {
    font_is_google <- paste0(font_type, "_font_is_google")
    values[[font_is_google]] <- if (!is.null(values[[font_is_google]])) {
      values[[font_is_google]] %in% c("1", "TRUE", "true", "yes")
    } else FALSE
  }
  values
}