Disable color picker when using system color

CHANGELOG.md

@@ -6,6 +6,7 @@
 ### Implements
 
 - Update the Markdown syntax to fix highlighting strikethrough.
+- Disable color wells in the theme editor when they use the system color.
 - Improve the User Guide contents.
 - [non-AppStore ver.] Update Sparkle from 2.6.2 to 2.6.3.
 

CotEditor/Sources/ThemeEditorView.swift

@@ -169,6 +169,7 @@ private struct SystemColorPicker: View {
                 Text(self.label)
                     .accessibilityLabeledPair(role: .label, id: "color", in: self.accessibility)
             }
+            .disabled(self.selection.usesSystemSetting)
             Toggle(String(localized: "Use system color", table: "ThemeEditor", comment: "toggle button label"), isOn: $selection.usesSystemSetting)
                 .controlSize(.small)
                 .accessibilityLabeledPair(role: .content, id: "color", in: self.accessibility)