[Theme: colormag] How can i edit post summary text?

You can try adding a manual excerpt. That is, when you go to edit a post, scroll down a little bit and you should see a separate field labeled Excerpt. If you don’t see it, click the Screen Options tab in the upper right corner of the screen and check the box labeled Excerpt and it should now appear. Copy & paste the text that you want to appear as the excerpt and hopefully it will appear on your front page in place of the text that the theme automatically selects.