Where to find classes for custom.css

[M.N]

Decided to use PS Classic theme, created child and started editing custom.css file. There is trouble to find correct elements to define. In example, it is working for the header  #header { background: gray; color: red;} where background is gray but only shopping basket text turns red. For contact, language  and currency block i can not find the class to define. Based on what can be found on the inspect element I have tested: #contact-link, #header-nav, #language-selector, #currency-selector, #menu-icon and some more all with simple definition  { color: red;} for testing purposes. However, nothing seems to work. So question is not specific, but general, as I need to change a lot more. What is the best method to find out correct class name for custom.css?

The only ones so far I could use have been header and footer and they work for background only...

[M.N]

Any ideas?

[Guest]

Each module has its own CSS style.
If you want to change any class or id, you must use! Important.
E.g. background: #fffff! important;

[M.N]

Thank you for tip, D.T! Finally resolved when I open from Inspect element the format on the right column and go directly to css file. Then copy the exact styling reference to my custom css. After that it works every time. Only few things left, that are difficult to find but this is already small problem. Case closed.