For changing the article text, you need to create a CSS rule targetting the “article-content” class. Your CSS will be more or less like this:
.article-content {
// your CSS properties
}
This will change the way the text of your posts is presented. If you want to change other elements in your page, you have to figure out what CSS classes are being applied to them or what identifiers are being used. To do this, in your page, right-click on the element you want to style and select the “Inspect Element” option. A panel with your HTML will appear, along with another panel, on the right, showing the CSS rules that apply to the element you clicked on. Browse through these panels to figure out the identifier (id) being used or the classes you want to change.
If you are new to CSS, here is a good tutorial to get you started: https://www.htmldog.com/guides/css/beginner/
Just remember: You should not change the files in your theme, since any changes you made will be gone as soon as you update the theme. To customize your theme, create a child theme.