gui: Use nested namespace for theme name translation keys (#9220)

Following up on #9192, this makes use of the new mechanism for the theme
names.

The dummy string added for testing is removed again here. All
translations are updated to the new nested syntax, except Chinese
(zh-HK) where the string weren't actually translated.

gui/default/assets/lang/lang-zh-CN.json

@@ -538,10 +538,14 @@
     "modified": "已修改",
     "permit": "允许",
     "seconds": "秒",
-    "theme-name-black": "黑色",
-    "theme-name-dark": "深色",
-    "theme-name-default": "默认",
-    "theme-name-light": "浅色",
+    "theme": {
+        "name": {
+            "black": "黑色",
+            "dark": "深色",
+            "default": "默认",
+            "light": "浅色"
+        }
+    },
     "{%device%} wants to share folder \"{%folder%}\".": "{{device}} 想将 “{{folder}}” 文件夹共享给您。",
     "{%device%} wants to share folder \"{%folderlabel%}\" ({%folder%}).": "{{device}} 想要共享 \"{{folderlabel}}\" ({{folder}}) 文件夹给您。",
     "{%reintroducer%} might reintroduce this device.": "{{reintroducer}}可能会重新引入此设备。"