How to make corners round?

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https://preview.redd.it/xnbvoifj3gad1.jpg?width=1920&format=pjpg&auto=webp&s=a032ae4430527bab11bc94f7dd0ddc892b8e945b

Using a theme by StudioPress. Do I need to access HTML code this page and make changes? If so, any clue how to access corresponding HTML code while using a block-based editor? Earlier in Classic Editor, there used to be two options: Visual (Text) and HTML right on the editor itself.