feat: Use primary color specified in config

Sorry, not sorry iPhone users, get a phone on which you are able to use something different than Safari, or get a newer one :)

config.toml

@@ -23,9 +23,11 @@ highlight_themes_css = [
 smart_punctuation = true
 
 [extra]
-# Sets browser theme color,
+# Sets theme and browser theme color,
 # see https://developer.mozilla.org/en-US/docs/Web/HTML/Element/meta/name/theme-color
 primary_color = "#ff7800"
+# Same as primary_color, but with 20% opacity
+primary_color_alpha = "rgba(255, 120, 0, 0.2)"
 # If the favicon is GIF or not
 animated_favicon = false
 # Tera date format (not used in comments!),