How to add a button in my case?

I think I follow you,
may I ask if you are using a Child Theme, Custom Theme or Custom Style plugin?

I think it is a custom theme from elegantthemes.com.Name of the theme is Glow

Can you create a child-theme of your custom theme, or use the Custom Style plugin?

Child theme – https://codex.www.ads-software.com/Child_Themes
Custom Style – https://www.ads-software.com/extend/plugins/custom-style/

It’s just you don’t want to be editing the theme’s CSS; when the theme is updated, your changes will be lost.

I installed Custom Style plugin.Is that what I should see?
https://www.imagebam.com/image/25df39205680213

Yes that is correct.

Do you know any CSS?
If not, can you link us your website or Pastebin.com the HTML and CSS code of that page relevant?

Yes I know some CSS. I can send you all theme files if you want because I don’t know exactly which .php file I should send.

To get the HTML,
Right-click the web page (assuming you’re on windows) and select “view source”.

Try just this for now.

https://pastebin.com/31fwdKP1

css

https://pastebin.com/LxfYvqhT

I figured out how to do it myself. So thanks for help anyway