Fix themes to support primary search/selection

[nazarewk]

I thought it was [something wrong with helix install](see also helix-editor/helix#5309), but looks like numerous upstream themes have this issue too helix-editor/helix#3842 (including default theme).

this seems to be refered to as highlighting for the primary cursor

here seems to be an example fix https://github.com/helix-editor/helix/pull/5573/files

[nazarewk]

actually this is explicitly set to the same as normal selection

base16-helix/templates/default.mustache

Lines 62 to 63 in 2b75de9

	"ui.selection" = { bg = "base02" }
	"ui.selection.primary" = { bg = "base02" }

it should be different to even go through hx --tutor

[nazarewk]

another option is distinguishing cursor for selections:

"ui.cursorline" = { bg = "my_gray3" }
"ui.cursorline.primary" = { bg = "my_black" }
"ui.cursorline.secondary" = { bg = "my_black" }

[JamyGolden]

@krgn (tagging you since github doesn't auto tag maintainers people on github issues)