Improve onedarker theme contrast cursorline/selection (#12579)

Previously `ui.cursorline.primary`
(`light-black`)/`ui.selection.primary` (`gray`) and `ui.selection`
(`light-gray`)/`comment` (`light-gray`) had little or no contrast.
`ui.cursorline.primary` is changed to a new darker color, `dark-black`
and `ui.selection` is set to use `reversed`.

runtime/themes/onedarker.toml

@@ -65,9 +65,9 @@
 "ui.cursor.primary" = { fg = "white", modifiers = ["reversed"] }
 "ui.cursor.match" = { fg = "blue", modifiers = ["underlined"]}
 
-"ui.selection" = { bg = "light-gray" }
+"ui.selection" = { bg = "light-gray", modifiers = ["reversed"] }
 "ui.selection.primary" = { bg = "gray" }
-"ui.cursorline.primary" = { bg = "light-black" }
+"ui.cursorline.primary" = { bg = "dark-black" }
 
 "ui.linenr" = { fg = "linenr" }
 "ui.linenr.selected" = { fg = "white" }
@@ -105,6 +105,7 @@ gold = "#D19A66"
 cyan = "#46A6B2"
 white = "#ABB2BF"
 black = "#16181A"
+dark-black = "#12100E"
 light-black = "#2C323C"
 gray = "#252D30"
 faint-gray = "#ABB2BF"