Grep the cascade before you write the override
One of our sites runs a fixed starfield behind the whole page. Sections scroll over it, and we wanted their backgrounds transparent so the stars show through. Easy fix. We appended an override, background transparent on every section, and reloaded.
One section stayed solid black. It blacked out the stars as it passed over them. We read the override five times.
Right selector, right property, right value, no typo. It did nothing, and the browser gave no warning, no error, nothing in the console.
The rule that beat us sat 46 lines away
Long-lived stylesheets grow the way ours did: block by block, each feature or fix appended at the end with a dated comment. Our transparency override landed at line 243. At line 289 sat that section's own component block, added later, and it set its own background, a black gradient.
Both rules used the same selector at the same specificity. CSS breaks that tie by source order. The later rule wins, silently. Line 289 beat line 243, and our correct override lost a fight we did not know it was in.
A long stylesheet is a wall with 20 coats of paint. The color you see is whichever coat went on last, not the one you brushed hardest.
The 30-second discipline
Before appending any CSS override, grep every rule that touches the selector:
grep -n '\.the-selector' styles.css
Read each hit. If one rule sets the property, edit that rule directly and append nothing. If several rules touch the selector, the last one in source order owns the tie, and that is the rule to change.
That is what we shipped. We deleted the background declaration from the line 289 block instead of stacking a third rule on the pile. One rule owns the property now, and the override became unnecessary.
Two grep refinements earn their keep. Selectors hide in comma lists, so grep -nE '\.selector\s*\{|\.selector\s*,' catches both .selector { and .selector, .other {. Pseudo-elements need '\.selector(::|:)'.
Appending is still fine for a genuinely cross-cutting rule that targets many selectors at once. But grep first anyway, and either strip the conflicting declarations from the later blocks or place your rule after them. Position is part of the rule.
The !important trap
!important would have fixed our section in 5 seconds. It also raises the floor. The next change to that property needs its own !important, and the one after that needs a more specific selector on top. Each shortcut makes the following fix harder, until every rule in the file shouts and none of them can hear each other.
We skipped it, and we keep skipping it. The grep costs 30 seconds. Untangling a stylesheet full of !important costs an afternoon you will not get back.
The bug here was never CSS knowledge. We knew source order breaks specificity ties. The bug was writing the fix before reading the file, and trusting that the CSS we wrote was the CSS in effect.
Grep first, then write. Cheapest habit in the trade, and the stars show through every section now.