Bug 1755656 - [devtools] Updated fixing-bugs doc for CSS changes. r=Honza

added `mach build faster` command to doc to show local changes.

Differential Revision: https://phabricator.services.mozilla.com/D139228

devtools/docs/contributor/contributing/fixing-bugs.md

@@ -95,12 +95,19 @@ And that means it's time to ask for more information in the bug or from your col
 
 ### If this is about changing CSS code...
 
-If you think you have found the right file and the right CSS selector, you could try to edit the file to insert some outrageously colourful rule (for example, a really thick bright blue border), then restart DevTools to see if the changes show up.
+If you think you have found the right file and the right CSS selector, you could try to edit the file to insert some outrageously colourful rule (for example, a really thick bright blue border).
 
 ```css
 border: 4px solid blue;
 ```
 
+Check if the changes show up by rebuilding your local changes.
+
+```bash
+./mach build faster
+./mach run
+```
+
 ## NEXT: do whatever needs doing
 
 This will always depend on the specific bug you're working on, so it's hard to provide guidance here.