Fix: add Linux and custom font directories

docs/dev/analysis/txt-font-typeface.rst

@@ -10,6 +10,11 @@ Verdana to Arial. This aspect of a font is its *typeface*, as opposed to its
 size (e.g. 18 points) or style (e.g. bold, italic).
 
 
+.. admonition:: Custom fonts folder
+
+   |pp| will search for font files on each OS default directories. A list of custom folders can also be specified by setting the ``PYTHON_PPTX_FONT_DIRECTORY`` environment variable to a colon-separated list of the paths of each fonts folder.
+
+
 Minimum viable feature
 ----------------------
 

docs/user/install.rst

@@ -23,3 +23,14 @@ Dependencies
 * lxml
 * Pillow
 * XlsxWriter (to use charting features)
+
+
+.. admonition:: Note: font availability on Linux
+
+   On most Linux distros, default PowerPoint fonts are typically unavailable. Thus, you should install them manually by either:
+
+   - Installing ``ttf-mscorefonts-installer`` package (on Ubuntu).
+   - Copying font files from the ``C:\Windows\Fonts`` folder of an existing Windows machine.
+   - Manually installing the fonts following `this guide <https://wiki.debian.org/ppviewerFonts>`_.
+
+

src/pptx/text/fonts.py

@@ -43,11 +43,26 @@ def _font_directories(cls):
         Return a sequence of directory paths likely to contain fonts on the
         current platform.
         """
+
+        font_dirs = []
+
+        # Colon-separated list of paths for python-pptx to find fonts
+        CUSTOM_DIRS_VAR_NAME = 'PYTHON_PPTX_FONT_DIRECTORY'
+        custom_dirs = os.getenv(CUSTOM_DIRS_VAR_NAME)
+
+        # Check it is not None, empty or whitespace
+        if custom_dirs and custom_dirs.strip():
+            font_dirs.extend(custom_dirs.split(':'))
+
         if sys.platform.startswith("darwin"):
-            return cls._os_x_font_directories()
-        if sys.platform.startswith("win32"):
-            return cls._windows_font_directories()
-        raise OSError("unsupported operating system")
+            font_dirs.extend(cls._os_x_font_directories())
+        elif sys.platform.startswith("linux"):
+            font_dirs.extend(cls._linux_font_directories())
+        elif sys.platform.startswith("win32"):
+            font_dirs.extend(cls._windows_font_directories())
+        else:
+            raise OSError("unsupported operating system")
+        return font_dirs
 
     @classmethod
     def _iter_font_files_in(cls, directory):
@@ -84,6 +99,23 @@ def _os_x_font_directories(cls):
             )
         return os_x_font_dirs
 
+    @classmethod
+    def _linux_font_directories(cls):
+        """
+        Return a sequence of directory paths in which fonts are
+        likely to be located on most Linux.
+        """
+        linux_font_dirs = [
+            "/usr/share/fonts",
+            "/usr/local/share/fonts",
+        ]
+        home = os.environ.get("HOME")
+        if home is not None:
+            linux_font_dirs.extend(
+                [os.path.join(home, ".local", "share", "fonts"), os.path.join(home, ".fonts")]
+            )
+        return linux_font_dirs
+
     @classmethod
     def _windows_font_directories(cls):
         """