How to changed the "underline--bar" default style "tall" to "solid"?

[Yurii-huang]

 Tabs:focus .underline--bar {
        background: $foreground 20%;
    }

[TomJGooding]

I don't think it's possible to change the thickness/style of the underline bar, only the color. The actual glyphs used to create the bar are baked into the 'Bar' renderable used by the Tabs widget by the looks of it.

[Yurii-huang]

This default style is not very user-friendly on serial ports or other high-resolution TTYs

[Yurii-huang]

Do you have a good suggestion for changing the style？

[TomJGooding]

Nothing straightforward I'm afraid. You'd need to create a custom Bar renderable, then a custom Underline widget which renders your bar, then a custom Tabs widget composed with your custom underline.

That's all assuming you aren't using TabbedContent, which would require extra work.