Initial support for colorblindness-friendly themes (#30625)

Initial support for #25680

This PR only adds some simple styles from GitHub, it is big enough and
it focuses on adding the necessary framework-level supports. More styles
could be fine-tuned later.

services/context/context.go

@@ -230,6 +230,7 @@ func Contexter() func(next http.Handler) http.Handler {
 
 // HasError returns true if error occurs in form validation.
 // Attention: this function changes ctx.Data and ctx.Flash
+// If HasError is called, then before Redirect, the error message should be stored by ctx.Flash.Error(ctx.GetErrMsg()) again.
 func (ctx *Context) HasError() bool {
 	hasErr, ok := ctx.Data["HasError"]
 	if !ok {