diff --git a/.github/workflows/python-build.yaml b/.github/workflows/python-build.yaml
index b000923..1d0b845 100644
--- a/.github/workflows/python-build.yaml
+++ b/.github/workflows/python-build.yaml
@@ -21,10 +21,10 @@ jobs:
     - uses: actions/checkout@v4
     - name: Install ccache for Nuitka
       run: sudo apt update && sudo apt install -y ccache libicu70
-    - name: Set up Python 3.11
+    - name: Set up Python 3.12
       uses: actions/setup-python@v5
       with:
-        python-version: "3.11"
+        python-version: "3.12"
     - name: Install Dependencies
       run: |
         python -m pip install --upgrade pip
@@ -49,10 +49,10 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
-      - name: Set up Python 3.11
+      - name: Set up Python 3.12
         uses: actions/setup-python@v5
         with:
-          python-version: "3.11"
+          python-version: "3.12"
       - name: Install Dependencies
         run: |
           python -m pip install --upgrade pip
@@ -79,10 +79,10 @@ jobs:
     - uses: actions/checkout@v4
     - name: Enable Developer Command Prompt
       uses: ilammy/msvc-dev-cmd@v1.13.0
-    - name: Set up Python 3.11
+    - name: Set up Python 3.12
       uses: actions/setup-python@v5
       with:
-        python-version: "3.11"
+        python-version: "3.12"
     - name: Install Dependencies
       run: |
         python -m pip install --upgrade pip
diff --git a/commands/archive/theme.py b/commands/archive/theme.py
index 612dd6b..bbe29fe 100644
--- a/commands/archive/theme.py
+++ b/commands/archive/theme.py
@@ -1,4 +1,58 @@
 # "commands/archive/theme.py" from WiiPy by NinjaCheetah
 # https://github.com/NinjaCheetah/WiiPy
 
+import configparser
+import pathlib
+import shutil
+import tempfile
+import zipfile
+import libWiiPy
+import time
 
+
+def handle_apply_mym(args):
+    mym_path = pathlib.Path(args.mym)
+    base_path = pathlib.Path(args.base)
+    output_path = pathlib.Path(args.output)
+
+    if not mym_path.exists():
+        raise FileNotFoundError(mym_path)
+    if not base_path.exists():
+        raise FileNotFoundError(base_path)
+    if output_path.suffix != ".csm":
+        output_path = output_path.with_suffix(".csm")
+
+    with tempfile.TemporaryDirectory() as tmp_dir:
+        tmp_path = pathlib.Path(tmp_dir)
+        # Extract the MYM file into the temp directory.
+        # MYM files are just ZIP files, so if zipfile doesn't accept it then it can't be valid.
+        try:
+            with zipfile.ZipFile(mym_path) as mym:
+                mym.extractall(tmp_path.joinpath("mym_out"))
+        except zipfile.BadZipfile:
+            print("Error: The provided MYM theme is not valid!")
+            exit(1)
+        mym_tmp_path = pathlib.Path(tmp_path.joinpath("mym_out"))
+        # Extract the asset archive into the temp directory.
+        try:
+            libWiiPy.archive.extract_u8(base_path.read_bytes(), str(tmp_path.joinpath("base_out")))
+        except ValueError:
+            print("Error: The provided base assets are not valid!")
+            exit(1)
+        base_temp_path = pathlib.Path(tmp_path.joinpath("base_out"))
+        # Parse the mym.ini file in the root of the extracted MYM file.
+        mym_ini = configparser.ConfigParser()
+        mym_ini.read(mym_tmp_path.joinpath("mym.ini"))
+        # Iterate over every key in the ini file and apply the theme based the source and target of each key.
+        for section in mym_ini.sections():
+            source_file = mym_tmp_path
+            for piece in mym_ini[section]["source"].replace("\\", "/").split("/"):
+                source_file = source_file.joinpath(piece)
+            target_file = base_temp_path
+            for piece in mym_ini[section]["file"].replace("\\", "/").split("/"):
+                target_file = target_file.joinpath(piece)
+            shutil.move(source_file, target_file)
+        # Repack the now-themed asset archive and write it out.
+        output_path.write_bytes(libWiiPy.archive.pack_u8(base_temp_path))
+
+    print(f"Applied theme \"{mym_path.name}\" to \"{output_path.name}\"!")
diff --git a/wiipy.py b/wiipy.py
index 99520da..5aacf8f 100644
--- a/wiipy.py
+++ b/wiipy.py
@@ -5,6 +5,7 @@ import argparse
 from importlib.metadata import version
 
 from commands.archive.ash import *
+from commands.archive.theme import *
 from commands.archive.u8 import *
 from commands.nand.emunand import *
 from commands.nand.setting import *
@@ -196,6 +197,20 @@ if __name__ == "__main__":
     setting_gen_parser.add_argument("region", metavar="REGION", type=str,
                                            help="region of the console these settings are for (USA, EUR, JPN, or KOR)")
 
+    # Argument parser for the theme subcommand.
+    theme_parser = subparsers.add_parser("theme", help="apply custom themes to the Wii Menu",
+                                         description="apply custom themes to the Wii Menu")
+    theme_subparsers = theme_parser.add_subparsers(dest="subcommand", required=True)
+    # MYM theme subcommand.
+    theme_mym_parser = theme_subparsers.add_parser("mym", help="apply an MYM theme to the Wii Menu",
+                                                   description="apply an MYM theme to the Wii Menu")
+    theme_mym_parser.set_defaults(func=handle_apply_mym)
+    theme_mym_parser.add_argument("mym", metavar="MYM", type=str, help="MYM theme to apply")
+    theme_mym_parser.add_argument("base", metavar="BASE", type=str,
+                                  help="base Wii Menu assets to apply the theme to (000000xx.app)")
+    theme_mym_parser.add_argument("output", metavar="OUT", type=str,
+                                  help="path to output the finished theme to (<filename>.csm)")
+
     # Argument parser for the TMD subcommand.
     tmd_parser = subparsers.add_parser("tmd", help="edit a TMD file",
                                        description="edit a TMD file")