In my website, pagination is like this:

StartPrev123NextEnd

There is no space between words and numbers; no styling (like default Joomla).

Any help to correct this?

K2 uses default joomla styling, if your template does not have a styling, you need to add your own css.

You can also copy the k2.css file from the components directory to your template's css directory, then you can change the styling without messing the default k2.css

Nothing happened after adding this css. :(

I think there is some other class