Login in to Magento Admin
Go to Marketing -> Communications -> Email templates form Magento Navigation.
Now, click on Add New Template.
If you want to load default template than Select template and click on load template. After click in Template Information default content will be load.
You can add content on Template Content section and also give custom CSS that you can write on Template Styles.
You can see preview of your Email template by click on Preview Template.
And after all perfect Save Template.
For change Default Welcome Email (New customer)
Go to Stores -> configuration from Magento Navigation.
After that go to Customers -> Customer Configuration in that unchecked Use system value of Default Welcome Email and select your created custom template from drop box.
Save Configuration by click on Save Config button.