Please review the following checklist.

• [ N/A ] Docstrings on all new or modified functions / classes
• [ N/A ] Updated documentation
• Updated CHANGELOG.md (where appropriate)

I believe I've found the fix for this issue as originally reported in the Discord help channel.

So first off, original MRE showcasing the problem since, as requested, we're going straight to PR without an attached issue:

from textual.app import App
from textual.widgets import Switch, Static
from textual.containers import Container

class TextualApp(App[None]):

    CSS = """
    Screen { background: darkblue; align: center middle; }
    #my_container {
        width: 50%; height: 50%;
        align: center middle;
        border: solid yellow;
        background: red;
        &.-transparent { background: transparent; }        
    }
    """

    def compose(self):
        with Container(id="my_container"):
            yield Static("Make container transparent:")
            yield Switch()

    def on_switch_changed(self, event: Switch.Changed):

        my_container = self.query_one("#my_container")
        if event.value:
            my_container.styles.background = "transparent"  # before fix: this doesn't work
            # my_container.add_class("-transparent")     # adding a CSS class works fine however
        else:
            my_container.styles.background = "red"
            # my_container.remove_class("-transparent")

TextualApp().run()

The issue was that in the ColorProperties class in css/_style_properties.py, it was not accounting for the case where a named color might have alpha built into it. Which is of course only one named color, "transparency".

Line in question:

In the fix, I've added an extra line:

      try:
          parsed_color = Color.parse(token)
          _r, _g, _b, alpha, _, _ = parsed_color    # new line

Previously it would only get the alpha if token.endswith("%"), which was missing the edge case where the string is "transparent". Now after parsing the string, it extracts whatever alpha value is to re-use it for the parsed_color.with_alpha(alpha) call.

I thought this was a nice solution since its a one-line addition, which is a bit more elegant than making an extra condition just for the "transparent" string. But regardless, I'm pretty sure this is the root of the issue. Let me know if you think this is maybe not the best way to fix it.