[Bug]: Cannot set transparency whatsoever. Defaults to terminal background color.

### Is there an existing issue for this?

- [x] I have searched the existing issues

### What happened?

I'm using nvim on windows with the terminal from microsoft store.

i've tried manually setting to none using cmdline `:lua nvim.api.nvim_set_hl(...)`
It seems to be defaulting to what I've configured on the terminal and I can't seem to get transparency whatsoever.

I've seen the [help issue asking how to set transparency](https://github.com/akinsho/bufferline.nvim/issues/1021) but there wasn't anything useful
I just want to fully see cute anime girls while I program, man. 😔

### What did you expect to happen?

Transparency

### Config
```
require('bufferline').setup({
options = {
custom_filter = function(buf_num, _)
return vim.bo[buf_num].buftype ~= "terminal"
end
},
highlights = {
fill = {
bg = 'none',
},
background = {
bg = 'none',
},
buffer_visible = {
bg = 'none',
},
buffer_selected = {
bg = 'none',
fg = '#16a5e4'
}
}
})
```
### Additional Information

...

### commit

655133c

Start by reproducing the configuration in the issue on Neovim for Windows, focusing on the `require('bufferline').setup` highlights entry point and comparing it with `nvim.api.nvim_set_hl(...)`. No file or test is named in the report; done means the listed bufferline highlight groups render with `bg = 'none'` instead of the terminal background, with regression coverage for the behavior.

索引モデルが issue の本文から書いたものです。