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.

spec/components/validators/css_color_validator_spec.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe CssColorValidator do
+  subject { described_class.new }
+
+  it "validates hex colors" do
+    expect(subject.valid_value?('#0')).to eq(false)
+    expect(subject.valid_value?('#00')).to eq(false)
+    expect(subject.valid_value?('#000')).to eq(true)
+    expect(subject.valid_value?('#0000')).to eq(false)
+    expect(subject.valid_value?('#00000')).to eq(false)
+    expect(subject.valid_value?('#000000')).to eq(true)
+  end
+
+  it "validates css colors" do
+    expect(subject.valid_value?('red')).to eq(true)
+    expect(subject.valid_value?('green')).to eq(true)
+    expect(subject.valid_value?('blue')).to eq(true)
+    expect(subject.valid_value?('hello')).to eq(false)
+  end
+end