Which index.css or custom.css is the right one to be modified?

I just want to change the color of bold texts in the default light theme, just as a workaround for making simple highlights to avoid typing in HTML tags.
The solution code snippet seems to be quite simple, strong {color: #xxxxxx;} or .jp-RenderedHTMLCommon strong {color: #xxxxxx;}, even for people like me who have little experience in CSS.
However, I cannot find the right CSS file to modify. I’ve searched many answers on StackOverflow but no one worked, because there are so many files named custom.css or index.css, and I tried to add the above code snippet in each of them but finally failed with no effect.
I’d appreciate it if someone could give me an example of the directory where the right file expected to be modified actually locates. Thanks.

I have the exact same problem. I have a custom.css file that works well for my personal computer. I want to replace the custom.css file in my work computer with the one on my personal computer, but becasue there are many folders with custom.css, i could not figure out which one should be replace.