theme(onedark): Remove bg for window separator (#3011)

Author: sudormrfbin

runtime/themes/onedark.toml

@@ -71,7 +71,7 @@ diagnostic = { modifiers = ["underlined"] }
 
 "ui.help" = { bg = "gray" }
 "ui.popup" = { bg = "gray" }
-"ui.window" = { bg = "gray" }
+"ui.window" = { fg = "gray" }
 "ui.menu" = { fg = "white", bg = "gray" }
 "ui.menu.selected" = { fg = "black", bg = "blue" }
 "ui.menu.scroll" = { fg = "white", bg = "light-gray" }