A style that does not apply is usually one of three things: the selector does not match, another rule wins, or the file is not loaded. Each has a check that answers it in seconds, and guessing is slower than running the three checks.
About 1 min of reading
Diagnose a rule that does not apply.
Loud test
Before you run it, say out loud what it will print. Then open the answer.
Match the real class
Before you run it, say out loud what it will print. Then open the answer.
Change a single property to a loud value, confirm it applies, then change the real value. Debugging one property at a time is faster than reasoning about a block of six that might have two problems.
The browser shows which rules matched an element and which lost. Reading that list is faster than reasoning about specificity, and it is the difference between a guess and an answer.
Change the value to something loud and see if anything moves. If nothing changes, the rule is not reaching the element, and the problem is the selector or the file, not the value.
Two rules on the same element resolve by specificity and order, and the winner is the one you did not intend. Reducing the specificity of your selector, or scoping it more narrowly, is the fix rather than forcing a value.
A stylesheet that is cached from an earlier version shows the old look. Changing the file and seeing no change is the cache, and the fix is a version parameter on the URL, not a bigger change.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.