Hello @anishnagar,
Thanks for asking. 
Well, there is no need to add CSS class to each and every Headline element. You can add a class to the section and then make the necessary changes.
- Inspect the section and under Customize > Setup > Class add the class name.
- Add following CSS under X/Pro > Theme Options > CSS:
.section-headline .x-text-headline h1 {
color: #ddd !important;
font-size: 2em !important;
}
In above CSS code section-headline
is the class name that I have assigned to the section. Along with that please replace class name as per your requirement. Above code will change the font size and color for H1 tag. In case you are using a different heading, please replace the same in code.
If you would like to learn CSS, I am sharing few resources that you take a look to get started with CSS and an interesting tool that you can use to speed up the development process.
I recommend you to watch following video that will help you to get started with CSS.
https://www.youtube.com/watch?v=MFR4WXiLzpc
Sometimes it can get a bit difficult to find out the right selector to be able to write the required CSS codes. A handy tool that can help you in this is Google Chrome dev tools. I am sharing the resource that you can refer to get started with dev tools.
https://developers.google.com/web/tools/chrome-devtools/css/
https://developers.google.com/web/tools/chrome-devtools/
https://www.youtube.com/watch?v=tP_kXBJWPhQ&t=200s
Thanks.