How to remove space between page title and content

Try using a browser developer tool for this kind of exploratory CSS work.
I use Google Chrome’s built-in Developer Tool because I find it particularly easy to use.

Alternatively, ask general CSS forums;
https://csscreator.com/forum
https://www.cssforum.com.pk/
https://css-tricks.com/forums/
https://www.welovecss.com/
https://www.codingforums.com/

Now that you know the style, from being exposed/added by the Developer Tool, you can add a new style to override it.

You ought to add new styles externally, not within the theme’s files. Changes to the theme’s files will compromise the theme and be compromised when the theme updates.

External changes can be made using a Child Theme or through a plugin that provides Custom CSS functionality.

I recommend using the Jetpack plugin and its Custom CSS functionality to hold your CSS modifications.

What is “child theme” and what if I use the theme provided by wordpress by modifying it.

Edit: Fixed link.

A child theme is a way to make changes to that WordPress provided theme and keep those changes when the parent theme is updated.

If you modify any of the code that comes with WordPress (such as that theme) then when the next update comes out you risk losing all of those changes. By creating and using a child theme you prevent that risk.

This also applies to themes downloaded from WordPress.

Please tell me how to use child theme? I mean how to create and upload child theme.

Gah! More coffee for me. I copied Andrew’s link and didn’t check it. This is the link you want that will explain how to create and use a child theme.

https://codex.www.ads-software.com/Child_Themes