it looks like that your theme is outdated. You should properly merge the XF latest version templates properly in your current forum theme as the updates for the same is pushed by xenforo team!
We help many in fixing these kind of issues. If you need such support you can message me!
Go to style properties > Emails
From there you can change the header text color and also try sending a test mail to yourself. This way you can make sure the colors are appropriate.
@Veer please fix these issue in this addon
In Event entity title you are not limiting the title to any number of characters, Which led to many titles to be larger and sometimes it throws error
In category Entity you are limiting it to 100 chars
Now I have many events having title...
You should consider using this addon to prevent spam users from registration : https://xenforo.com/community/threads/ozzmodz-verify-email-before-registration-paid.190510/
Other than that if you need something with automation to automatically soft delete content matching a certain criteria dm...