feat: add background highlight to LSP signature help

I used surface1 as the color, the same as those used by
LspReferenceText, LspReferenceRead, and LspReferenceWrite.

lua/catppuccin/groups/integrations/native_lsp.lua

@@ -75,7 +75,7 @@ function M.get()
 		LspDiagnosticsDefaultWarning = { fg = warning }, -- Used as the mantle highlight group. Other LspDiagnostic highlights link to this by default (except Underline)
 		LspDiagnosticsDefaultInformation = { fg = info }, -- Used as the mantle highlight group. Other LspDiagnostic highlights link to this by default (except Underline)
 		LspDiagnosticsDefaultHint = { fg = hint }, -- Used as the mantle highlight group. Other LspDiagnostic highlights link to this by default (except Underline)
-		LspSignatureActiveParameter = { fg = C.peach },
+		LspSignatureActiveParameter = { bg = C.surface1, fg = C.peach },
 		-- LspDiagnosticsFloatingError         = { }, -- Used to color "Error" diagnostic messages in diagnostics float
 		-- LspDiagnosticsFloatingWarning       = { }, -- Used to color "Warning" diagnostic messages in diagnostics float
 		-- LspDiagnosticsFloatingInformation   = { }, -- Used to color "Information" diagnostic messages in diagnostics float