Bug: Extension is erasing user's custom colorCustomizations

For testing, that's the user's original config:

    "workbench.activityBar.visible": false,
    "workbench.colorCustomizations": {
        "activityBar.background": "#111",
        "editor.background": "#000",
        "editor.lineHighlightBackground": "#333",
        "editorGroupHeader.tabsBackground": "#111",
        "editorIndentGuide.background": "#111",
        "editorLineNumber.activeForeground": "#fff",
        "editorWidget.background": "#111",
        "panel.background": "#111",
        "peekViewEditor.background": "#111",
        "sideBar.background": "#111",
        "sideBar.foreground": "#fff",
        "sideBarSectionHeader.background": "#000",
        "statusBar.background": "#111",
        "tab.activeBackground": "#000",
        "tab.inactiveBackground": "#111",
        "titleBar.activeBackground": "#111"
    },
    "workbench.colorTheme": "Atom One Dark",
    "workbench.editor.enablePreview": false,
    "workbench.editor.highlightModifiedTabs": true,
    "workbench.fontAliasing": "antialiased",
    "workbench.iconTheme": "vscode-icons",
    "workbench.startupEditor": "welcomePage",
  }```