colored box

I think you can use the Shortcodes Ultimate plugin to insert various types of contents (boxes, buttons, notes, spoilers etc.)

I want it to look more like the theme

Which theme are we talking about here? If your theme has colored boxes in its screenshot or demo then the designers must have offered an easy way for you to add boxes on your site. If this is indeed the case then you should read your theme’s documentation or contact the developer.