Various tweaks to theme, more code coverage

tests/test_toolbar_private.py

@@ -0,0 +1,23 @@
+from bouquin.toolbar import ToolBar
+
+
+def test_style_letter_button_handles_missing_widget(qtbot):
+    tb = ToolBar()
+    qtbot.addWidget(tb)
+    # Create a dummy action detached from toolbar to force widgetForAction->None
+    from PySide6.QtGui import QAction
+
+    act = QAction("X", tb)
+    # No crash and early return
+    tb._style_letter_button(act, "X")
+
+
+def test_style_letter_button_sets_tooltip_and_accessible(qtbot):
+    tb = ToolBar()
+    qtbot.addWidget(tb)
+    # Use an existing action so widgetForAction returns a button
+    act = tb.actBold
+    tb._style_letter_button(act, "B", bold=True, tooltip="Bold")
+    btn = tb.widgetForAction(act)
+    assert btn.toolTip() == "Bold"
+    assert btn.accessibleName() == "Bold"