add rainbow palette to the default theme

Author: the-mikedavis

theme.toml

@@ -81,6 +81,8 @@ error = "apricot"
 info = "delta"
 hint = "silver"
 
+rainbow = ["#fc6b59", "#fabd2f", "#b8bb26", "#8ec07c", "#83a598", "#d3869b", "#b16286"]
+
 [palette]
 white = "#ffffff"
 lilac = "#dbbfef"