Infra: Capitalisation Consistency (#6296)

* Rename Ryujinx.UI.Common

* Rename Ryujinx.UI.LocaleGenerator

* Update in Files

AboutWindow

* Configuration State

* Rename projects

* Ryujinx/UI

* Fix build

* Main remaining inconsistencies

* HLE.UI Namespace

* HLE.UI Files

* Namespace

* Ryujinx.UI.Common.Configuration.UI

* Ryujinx.UI.Common,Configuration.UI Files

* More instances

src/Ryujinx/UI/Helper/ThemeHelper.cs

@@ -0,0 +1,36 @@
+using Gtk;
+using Ryujinx.Common;
+using Ryujinx.Common.Logging;
+using Ryujinx.UI.Common.Configuration;
+using System.IO;
+
+namespace Ryujinx.UI.Helper
+{
+    static class ThemeHelper
+    {
+        public static void ApplyTheme()
+        {
+            if (!ConfigurationState.Instance.UI.EnableCustomTheme)
+            {
+                return;
+            }
+
+            if (File.Exists(ConfigurationState.Instance.UI.CustomThemePath) && (Path.GetExtension(ConfigurationState.Instance.UI.CustomThemePath) == ".css"))
+            {
+                CssProvider cssProvider = new();
+
+                cssProvider.LoadFromPath(ConfigurationState.Instance.UI.CustomThemePath);
+
+                StyleContext.AddProviderForScreen(Gdk.Screen.Default, cssProvider, 800);
+            }
+            else
+            {
+                Logger.Warning?.Print(LogClass.Application, $"The \"custom_theme_path\" section in \"{ReleaseInformation.ConfigName}\" contains an invalid path: \"{ConfigurationState.Instance.UI.CustomThemePath}\".");
+
+                ConfigurationState.Instance.UI.CustomThemePath.Value = "";
+                ConfigurationState.Instance.UI.EnableCustomTheme.Value = false;
+                ConfigurationState.Instance.ToFileFormat().SaveConfig(Program.ConfigurationPath);
+            }
+        }
+    }
+}