Fix issue with tibble 3.0.0

Closes #32

R/theme_settings.R

   , "header_color", "#000", "h1, h2, h3", "Header Color"
   , "background_color", "#FFF", ".remark-slide-content", "Slide Background Color"
   , "link_color", "rgb(249, 38, 114)", "a, a > code", "Link Color"
-  , "text_bold_color", NULL, "strong", "Bold Text Color"
+  , "text_bold_color", "{NULL}", "strong", "Bold Text Color"
   , "text_slide_number_color", "{inverse_background_color}", ".remark-slide-number", "Slide Number Color"
   , "padding", "16px 64px 16px 64px", ".remark-slide-content", "Slide Padding in `top right [bottom left]` format"
-  , "background_image", NULL, ".remark-slide-content", "Background image applied to each *and every* slide. Set `title_slide_background_image = \"none\"` to remove the background image from the title slide"
-  , "background_size", NULL, ".remark-slide-content", "Background image size, requires `background_image` to be set. If `background_image` is set, `background_size` will default to `cover` so the backround fills the screen. If both `background_image` and `background_position` are set, will default to 100 percent"
-  , "background_position", NULL, ".remark-slide-content", "Background image position, requires `background_image` to be set, and it is recommended to adjust `background_size`"
+  , "background_image", "{NULL}", ".remark-slide-content", "Background image applied to each *and every* slide. Set `title_slide_background_image = \"none\"` to remove the background image from the title slide"
+  , "background_size", "{NULL}", ".remark-slide-content", "Background image size, requires `background_image` to be set. If `background_image` is set, `background_size` will default to `cover` so the backround fills the screen. If both `background_image` and `background_position` are set, will default to 100 percent"
+  , "background_position", "{NULL}", ".remark-slide-content", "Background image position, requires `background_image` to be set, and it is recommended to adjust `background_size`"
   , "code_highlight_color", "rgba(255,255,0,0.5)", ".remark-code-line-highlighted", "Code Line Highlight"
   , "code_inline_color", "#000", ".remark-inline-code", "Inline Code Color"
-  , "code_inline_background_color", NULL, ".remark-inline-code", "Inline Code Background Color"
+  , "code_inline_background_color", "{NULL}", ".remark-inline-code", "Inline Code Background Color"
   , "code_inline_font_size", "1em", ".remark-inline-code", "Inline Code Text Font Size"
   , "inverse_background_color", "#272822", ".inverse", "Inverse Background Color"
   , "inverse_text_color", "#d6d6d6", ".inverse", "Inverse Text Color"
   , "inverse_header_color", "#f3f3f3", ".inverse h1, .inverse h2, .inverse h3", "Inverse Header Color"
   , "title_slide_text_color", "{inverse_text_color}", ".title-slide", "Title Slide Text Color"
   , "title_slide_background_color", "{inverse_background_color}", ".title-slide", "Title Slide Background Color"
-  , "title_slide_background_image", NULL, ".title-slide", "Title Slide Background Image URL"
-  , "title_slide_background_size", NULL, ".title-slide", "Title Slide Background Image Size, defaults to \"cover\" if background image is set"
-  , "title_slide_background_position", NULL, ".title-slide", "Title Slide Background Image Position"
-  , "footnote_color", NULL, ".footnote", "Footnote text color (if `NA`, then it will be the same color as `text_color`)"
+  , "title_slide_background_image", "{NULL}", ".title-slide", "Title Slide Background Image URL"
+  , "title_slide_background_size", "{NULL}", ".title-slide", "Title Slide Background Image Size, defaults to \"cover\" if background image is set"
+  , "title_slide_background_position", "{NULL}", ".title-slide", "Title Slide Background Image Position"
+  , "footnote_color", "{NULL}", ".footnote", "Footnote text color (if `NA`, then it will be the same color as `text_color`)"
   , "footnote_font_size", "0.9em", ".footnote", "Footnote font size"
   , "footnote_position_bottom", "60px", ".footnote", "Footnote location from bottom of screen"
   , "left_column_subtle_color", "#777", ".left-column h2, .left-column h3", "Left Column Text (not last)"
   , "header_background_auto", "{FALSE}", "", "Add background under slide title automatically for h1 header elements. If not enabled, use `class: header_background` to enable"
   , "header_background_color", "{header_color}", ".remark-slide-content h1", "Background Color for h1 Header with Background"
   , "header_background_text_color", "{background_color}", ".remark-slide-content h1", "Text Color for h1 Header with Background"
-  , "header_background_padding", NULL, ".remark-slide-content h1", "Padding for h1 Header with Background"
+  , "header_background_padding", "{NULL}", ".remark-slide-content h1", "Padding for h1 Header with Background"
   , "header_background_content_padding_top", "7rem", ".remark-slide-content", "Top Padding for Content in Slide with Header with Background"
   , "header_background_ignore_classes", "{c('normal', 'inverse', 'title', 'middle', 'bottom')}", ".remark-slide-content", "Slide Classes Where Header with Background will not be Applied"
   , "text_slide_number_font_size", "0.9em", ".remark-slide-number", "Slide Number Text Font Size"
-  , "text_font_google", NULL, "body", "Use `google_font()` to specify body font"
+  , "text_font_google", "{NULL}", "body", "Use `google_font()` to specify body font"
   , "text_font_family", "Noto Sans", "body", "Body Text Font Family (xaringan default is `'Droid Serif'`)"
   , "text_font_weight", "normal", "body", "Body Text Font Weight"
   , "text_font_url", "https://fonts.googleapis.com/css?family=Noto+Sans:400,400i,700,700i&display=swap", "@import url()", "Body Text Font URL(s)"
   , "text_font_family_fallback", "-apple-system, BlinkMacSystemFont, avenir next, avenir, helvetica neue, helvetica, Ubuntu, roboto, noto, segoe ui, arial", "body", "Body Text Font Fallbacks"
   , "text_font_base", "sans-serif", "body", "Body Text Base Font (Total Failure Fallback)"
-  , "header_font_google", NULL, "body", "Use `google_font()` to specify header font"
+  , "header_font_google", "{NULL}", "body", "Use `google_font()` to specify header font"
   , "header_font_family", "Cabin", "h1, h2, h3", "Header Font Family (xaringan default is `'Yanone Kaffeesatz'`)"
   , "header_font_weight", "600", "h1, h2, h3", "Header Font Weight"
   , "header_font_url", "https://fonts.googleapis.com/css?family=Cabin:600,600i&display=swap", "@import url", "Header Font URL"
-  , "code_font_google", NULL, "body", "Use `google_font()` to specify code font"
+  , "code_font_google", "{NULL}", "body", "Use `google_font()` to specify code font"
   , "code_font_family", "Source Code Pro", ".remark-code, .remark-inline-code", "Code Font Family"
   , "code_font_size", "0.9rem", ".remark-inline", "Code Text Font Size"
   , "code_font_url", "https://fonts.googleapis.com/css?family=Source+Code+Pro:400,700&display=swap", "@import url", "Code Font URL"