[FEAT]: Stackbar tab styling

[CtByte]

Is your feature request related to a problem? Please describe.
When I have a background close to the background color of the stackbar.tabs, it is difficult to see the stackbar tabs.

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Describe the solution you'd like
A. If there is a border set with the same color as the focused_text and unfocused_text, would be enough for the tabs to stand out more.

B. An other idea could be to add focused_border, unfocused_border, focused_background, unfocused_background (maybe use the background to set both focused and unfocused backgrounds at the same time).

C. Could organize the setting like this, but it could break old configs:

  "stackbar": {
    "height": 40,
    "label": "Title",
    "mode": "OnStack",
    "tabs": {
      "width": 300,
      "focused": {
        "text": "",
        "border": "",
        "background": "",
      },
      "unfocused": {
        "text": "",
        "border": "",
        "background": "",
      }
    }
  },

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Describe alternatives you've considered
I know that I could change the stackbar.tabs.background color, but I would like to keep the dark theme as my windows.

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Additional context
After seeing your video on the undocumented win32 pitfall, I hope this wouldn't complicate things.