Global mode string is not visible on the tab bar

### What did you expect to happen?

When adding the `tab-bar-format-global` function to the `tab-bar-format` list, the global mode string should be visible in the tab bar.

### What actually happened?

The global mode string is not visible.

### Describe your attempts to resolve the issue

The issue can be resolved by changing the face definition of `tab-line`.
Currently, `tab-line` is define as:
```
(tab-line :background bg-alt :foreground bg-alt)
```

Changing the foreground color to a different color than the background one will make the global mode string visible.

### Steps to reproduce

Run this while any doom emacs theme is loaded:
```
(setq tab-bar-format '(tab-bar-format-tabs-groups
tab-bar-separator
tab-bar-format-align-right
tab-bar-format-global))

(display-time-mode 1)
```
I expect the clock to be shown in the upper right corner of the frame.

### Package commit

83fd9545c2823b4b2610947802fa3a52995517b4

### System Information

Emacs 28.1, on all platforms.

Reproduce the issue with the provided tab-bar-format and display-time-mode forms while a Doom Emacs theme is loaded. Start at the theme’s tab-line face definition and verify that changing its foreground makes the global mode string visible; done means the clock appears in the upper-right tab bar without reducing tab readability.

Written by the indexing model from the issue text.