Customize output style of views filter fields
The CSS classes given by Views should normally be good enough, if you don't know how to find them you may using a chrome or firefox browser: right click on the filter field and then inspect element

Or if you want you may install the Better exposed filters module and follow the intructions given in this question

UPDATE:

As per your comment below, how to group filter fields under one div ?, it is out of the scope of the original question and for that you will have to ask another question.

However, I believe that what you are asking for is not doable using the views UI and you will have to write your own custom codes, for that you may find the below links somehow useful even if they are referencing the D7 version:


Wrapping fields together in a views exposed filter
Place exposed filters in a fieldset
Views Grouping field to use a shared wrapper div
Views exposed groups Module for D7