Implement changes to ThemeHelper requested by review

- turned theme_attribute_map into frozen constant ATTRIBUTE_MAP
- early return if no theme exists, instead of if-block
- usage of Hash#key? instead Hash[k] to confirm existence of key in hash
- Early skip/next if key is not present in Hash instead of if-block

app/helpers/theme_helper.rb

@@ -1,6 +1,5 @@
 module ThemeHelper
-  def render_theme
-    theme_attribute_map = {
+  ATTRIBUTE_MAP = {
     'primary_color' => 'primary',
     'primary_text' => 'primary-text',
     'danger_color' => 'danger',
@@ -22,21 +21,23 @@ module ThemeHelper
     'input_color' => 'input-bg',
     'input_text' => 'input-text',
     'muted_text' => 'muted-text'
-    }
+  }.freeze
 
+  def render_theme
     theme = get_active_theme
 
-    if theme
+    return unless theme
 
     body = ":root {\n"
+
     theme.attributes.each do |k, v|
-        if theme_attribute_map[k]
+      next unless ATTRIBUTE_MAP.key?(k)
+
       if k.include? "text"
         hex = get_hex_color_from_theme_value(v)
-            body += "\t--#{theme_attribute_map[k]}: #{get_decimal_triplet_from_hex(hex)};\n"
+        body += "\t--#{ATTRIBUTE_MAP[k]}: #{get_decimal_triplet_from_hex(hex)};\n"
       else
-            body += "\t--#{theme_attribute_map[k]}: ##{get_hex_color_from_theme_value(v)};\n"
-          end
+        body += "\t--#{ATTRIBUTE_MAP[k]}: ##{get_hex_color_from_theme_value(v)};\n"
       end
     end
 
@@ -44,7 +45,6 @@ module ThemeHelper
 
     content_tag(:style, body)
   end
-  end
 
   def get_active_theme
     if @user&.theme