FIX: Validate email_accent_bg_color color (#13778)

Using an invalid value was allowed. This commit tries to automatically
fix the color by adding missing # symbol or will show an error to the
user if it is not possible and it is not a CSS color either.

lib/validators/css_color_validator.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+class CssColorValidator
+  COLORS = %w{
+    aliceblue antiquewhite aqua aquamarine azure beige bisque black
+    blanchedalmond blue blueviolet brown burlywood cadetblue chartreuse
+    chocolate coral cornflowerblue cornsilk crimson cyan darkblue darkcyan
+    darkgoldenrod darkgray darkgreen darkgrey darkkhaki darkmagenta
+    darkolivegreen darkorange darkorchid darkred darksalmon darkseagreen
+    darkslateblue darkslategray darkslategrey darkturquoise darkviolet
+    deeppink deepskyblue dimgray dimgrey dodgerblue firebrick floralwhite
+    forestgreen fuchsia gainsboro ghostwhite gold goldenrod gray green
+    greenyellow grey honeydew hotpink indianred indigo ivory khaki lavender
+    lavenderblush lawngreen lemonchiffon lightblue lightcoral lightcyan
+    lightgoldenrodyellow lightgray lightgreen lightgrey lightpink lightsalmon
+    lightseagreen lightskyblue lightslategray lightslategrey lightsteelblue
+    lightyellow lime limegreen linen magenta maroon mediumaquamarine
+    mediumblue mediumorchid mediumpurple mediumseagreen mediumslateblue
+    mediumspringgreen mediumturquoise mediumvioletred midnightblue mintcream
+    mistyrose moccasin navajowhite navy oldlace olive olivedrab orange
+    orangered orchid palegoldenrod palegreen paleturquoise palevioletred
+    papayawhip peachpuff peru pink plum powderblue purple red rosybrown
+    royalblue saddlebrown salmon sandybrown seagreen seashell sienna silver
+    skyblue slateblue slategray slategrey snow springgreen steelblue tan teal
+    thistle tomato turquoise violet wheat white whitesmoke yellow yellowgreen
+  }
+
+  def initialize(opts = {})
+    @opts = opts
+  end
+
+  def valid_value?(val)
+    !!(val =~ /^#([0-9a-fA-F]{3}|[0-9a-fA-F]{6})$/ || COLORS.include?(val&.downcase))
+  end
+
+  def error_message
+    I18n.t("site_settings.errors.invalid_css_color")
+  end
+end