Use one base font size and a per-profile startup design

JavaFX picks a 13px default font on macOS and Linux but 12px on
Windows, so labels in fixed-size controls were cut off outside
Windows. Both stylesheets now set 12px on the root.

Each operator profile now chooses light or dark mode for startup on
the GUI settings tab. The Windows menu entries remain a quick switch
for the running session. All windows, including the update window
and the top candidates list, follow the design, and the main window
no longer starts light while the station map starts dark.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

github_docs/en-Configuration.md

@@ -992,7 +992,9 @@ As before, the ON4KST password is stored in clear text in the `preferences.xml`
 
 ## Dark Mode (from v1.26)
 
-Enable Dark Mode through **Windows → Use dark mode design**. Use **Windows → Use default mode design** to restore the normal light colour scheme.
+Each operator profile sets its own startup colour scheme: on the **GUI** tab under **Design**, choose **Light mode** or **Dark mode**. The choice takes effect immediately and is stored in the profile's `preferences.xml` with **Save Settings**.
+
+For a quick change during operation, use **Windows → Use dark mode design** and **Windows → Use default mode design**. This switch only lasts until the next start or profile switch; after that the setting on the **GUI** tab applies again.
 
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