Add prefers-color-scheme support (#601)

* Add prefers-color-scheme support

This should fix <https://github.com/omarroth/invidious/issues/559>.
The cookie storage format has been changed from boolean
("true"/"false") to tri-state ("dark"/"light"/""), so that users
without a cookie set will get dark mode if they have enabled the dark
theme in their operating system. The code for handling the cookie
state, along with the user's operating system theme, has been factored
out into a new function `update_mode`, which is called both at window
load and at the "storage" event listener, because the "storage" event
listener is only trigerred when a change is made to the localStorage
from another tab/window (for more info - see
<https://stackoverflow.com/a/4679754>).

src/invidious/helpers/helpers.cr

@@ -24,6 +24,27 @@ end
 
 struct ConfigPreferences
   module StringToArray
+    def self.to_json(value : Array(String), json : JSON::Builder)
+      json.array do
+        value.each do |element|
+          json.string element
+        end
+      end
+    end
+
+    def self.from_json(value : JSON::PullParser) : Array(String)
+      begin
+        result = [] of String
+        value.read_array do
+          result << HTML.escape(value.read_string[0, 100])
+        end
+      rescue ex
+        result = [HTML.escape(value.read_string[0, 100]), ""]
+      end
+
+      result
+    end
+
     def self.to_yaml(value : Array(String), yaml : YAML::Nodes::Builder)
       yaml.sequence do
         value.each do |element|
@@ -44,11 +65,11 @@ struct ConfigPreferences
             node.raise "Expected scalar, not #{item.class}"
           end
 
-          result << item.value
+          result << HTML.escape(item.value[0, 100])
         end
       rescue ex
         if node.is_a?(YAML::Nodes::Scalar)
-          result = [node.value, ""]
+          result = [HTML.escape(node.value[0, 100]), ""]
         else
           result = ["", ""]
         end
@@ -58,6 +79,53 @@ struct ConfigPreferences
     end
   end
 
+  module BoolToString
+    def self.to_json(value : String, json : JSON::Builder)
+      json.string value
+    end
+
+    def self.from_json(value : JSON::PullParser) : String
+      begin
+        result = value.read_string
+
+        if result.empty?
+          CONFIG.default_user_preferences.dark_mode
+        else
+          result
+        end
+      rescue ex
+        result = value.read_bool
+
+        if result
+          "dark"
+        else
+          "light"
+        end
+      end
+    end
+
+    def self.to_yaml(value : String, yaml : YAML::Nodes::Builder)
+      yaml.scalar value
+    end
+
+    def self.from_yaml(ctx : YAML::ParseContext, node : YAML::Nodes::Node) : String
+      unless node.is_a?(YAML::Nodes::Scalar)
+        node.raise "Expected sequence, not #{node.class}"
+      end
+
+      case node.value
+      when "true"
+        "dark"
+      when "false"
+        "light"
+      when ""
+        CONFIG.default_user_preferences.dark_mode
+      else
+        node.value
+      end
+    end
+  end
+
   yaml_mapping({
     annotations:            {type: Bool, default: false},
     annotations_subscribed: {type: Bool, default: false},
@@ -66,7 +134,7 @@ struct ConfigPreferences
     comments:               {type: Array(String), default: ["youtube", ""], converter: StringToArray},
     continue:               {type: Bool, default: false},
     continue_autoplay:      {type: Bool, default: true},
-    dark_mode:              {type: Bool, default: false},
+    dark_mode:              {type: String, default: "", converter: BoolToString},
     latest_only:            {type: Bool, default: false},
     listen:                 {type: Bool, default: false},
     local:                  {type: Bool, default: false},

src/invidious/helpers/patch_mapping.cr

@@ -4,7 +4,7 @@ def Object.from_json(string_or_io, default) : self
   new parser, default
 end
 
-# Adds configurable 'default' to
+# Adds configurable 'default'
 macro patched_json_mapping(_properties_, strict = false)
   {% for key, value in _properties_ %}
     {% _properties_[key] = {type: value} unless value.is_a?(HashLiteral) || value.is_a?(NamedTupleLiteral) %}

src/invidious/helpers/utils.cr

@@ -356,3 +356,16 @@ def parse_range(range)
 
   return 0_i64, nil
 end
+
+def convert_theme(theme)
+  case theme
+  when "true"
+    "dark"
+  when "false"
+    "light"
+  when "", nil
+    nil
+  else
+    theme
+  end
+end