Support multiple CSS files (#1079)

* Support multiple CSS paths

* Update a type to match docstring

* Ensure the demo app still works

* Use absolute paths in tests to (hopefully) appease Windows

* Notes about CSS changes in guide/docstrings, small grammar/typos fixes

* Move snapshot apps into snapshot_tests dir, improve messaging in snapshot output, add test for multiple css files interacting with classvar CSS

* Ensure consistent snapshot naming cross-platform

* Use rpartition instead of partition in import_app

* Fix handling of import_app when colon in arg

* Support paths containing Windows drive names in import_app

* Add note on new relative paths in snap_compare

* Update docs/guide/CSS.md

Co-authored-by: Will McGugan <willmcgugan@gmail.com>

* Fix formatting

* Update CHANGELOG to mention CSS_PATH supporting a list

Co-authored-by: Will McGugan <willmcgugan@gmail.com>

tests/snapshot_tests/snapshot_apps/multiple_css/multiple_css.py

@@ -0,0 +1,34 @@
+"""Testing multiple CSS files, including app-level CSS
+
+-- element #one
+The `background` rule on #one tests a 3-way specificity clash between
+classvar CSS and two separate CSS files. The background ends up red
+because classvar CSS wins.
+The `color` rule tests a clash between loading two external CSS files.
+The color ends up as darkred (from 'second.css'), because that file is loaded
+second and wins.
+
+-- element #two
+This element tests that separate rules applied to the same widget are mixed
+correctly. The color is set to cadetblue in 'first.css', and the background is
+darkolivegreen in 'second.css'. Both of these should apply.
+"""
+from textual.app import App, ComposeResult
+from textual.widgets import Static
+
+
+class MultipleCSSApp(App):
+    CSS = """
+    #one {
+        background: red;
+    }
+    """
+
+    def compose(self) -> ComposeResult:
+        yield Static("#one", id="one")
+        yield Static("#two", id="two")
+
+
+app = MultipleCSSApp(css_path=["first.css", "second.css"])
+if __name__ == '__main__':
+    app.run()