hlgroups/NormalFloat/FloatBorder: Consider using the color sumiink1 instead of bg_dark

Currently, LSP diagnostics, using a floating style in my case, shows a background that does not match the original background. This occurs with other apps that use floating styles such as searchbox.nvim as shown in the issue #72.

• LSP diagnostic

• • Before
• • After

• SearchBox.nvim

• • Before
• • After

Hlgroups should use theme color names (i.e. bg), not palette colors.

This was a tough decision, but I preferred floating windows to have a darker background. It arguably looks bad with common borders tho.

Personally I use box characters that are drawn on the outer margin of the character grid. I'll give it a try and might reconsider tho.

This can be solved using the following configs, without changing the code:

	vim.api.nvim_set_hl(0, "NormalFloat", { bg = "#1F1F28", blend = 0})
	vim.api.nvim_set_hl(0, "FloatBorder", { bg = "#1F1F28"})
	vim.api.nvim_set_hl(0, "FloatTitle", { bg = "none"})