Hi,
Like so often, content and style are two separate things. So, one by one:
The content of these messages is typically in language files - as you were already guessing. So, to change it, you use language overrides. If you're not familiar with it, see the documentation of HikaShop ("
How to modify translations for my precise case
").
Usually, you can also use HTML in the text strings (careful with double quotes, though, they are used only to open and close a string - a simple workaround is using single quotes inside the strings where required). Means, you should be able to place icons and images as well.
The style, including fonts, sizes, etc, can be controlled per CSS.
Here some documentation
on CSS in general.