Yes, there is a lot of similar question to this one.
But I want to ask in the current time, is there any tool that can help to customize email reporting, either PDF or Alert?
Like to add color, HTML tags or any customizations.
There is a PDF tool that beats the pants off of the built-in one but I can't remember which one it is:
PDF Report Capture https://splunkbase.splunk.com/app/4132/
Smart PDF Exporter: https://splunkbase.splunk.com/app/4030/
As far as email in general, there is a bug-fix for whitespace in v8.0 which may help things look better but other than that, it is just a python script sendemail.py so you can easily go in and add/hardcode some stuff if you really have to.