visually distinct active tab and file(edited) status indicators

[txtyash]

In the above tabline from neovim you might be able to see a thin blue vertical line at the start of the active tab.
That is too thin for it's purpose but having a similar indicator or an underline under the active tab would be perceptible for quick grasp.

Regarding the "file edited" status which is shown using [ + ], could be replaced with something else that's again visually distinct and brighter.

[sudormrfbin]

The style of the active buffer can be configured with the ui.bufferline.active theme key and the background is controlled for other tabs are configured with ui.bufferline.background. Seems like this went undocumented in #2759 (\cc @aaron404).

[kirawi]

Fixed by #3690

[txtyash]

Not sure what #3690 exactly tries to fix, but half of this issue could be fixed by choosing better "bufferline colors" as I did in this PR. The other half is nitpicking.