When should I use !important, and why can it make CSS harder to change?

Not as the first move. Find the exact declarations applying to the button and compare them before changing anything. A later rule does not necessarily win just because it appears later: the cascade considers more than source order. !important marks a declaration important and changes its position in the cascade; it does not increase selector specificity. The full result also depends on cascade origin, layer, specificity, source order, and, in some cases, transitions. [S1]

Inspect the element’s computed styles in your browser’s developer tools, if available. Identify which rule supplies the blue color and whether your green rule is crossed out. Then compare the selectors and where each rule comes from. This gives you a reasoned fix instead of adding a declaration whose effect may be surprising.

Here is a hypothetical example. Assume the button is inside an element with id="site", and both rules apply to it. The first selector is more specific, so its blue color wins under otherwise equal cascade conditions.

Before:

#site button {
  color: blue;
}

.action-button {
  color: green;
}

Rather than adding !important, adjust the broad rule so it does not target action buttons:

After:

#site button:not(.action-button) {
  color: blue;
}

.action-button {
  color: green;
}

Now the blue rule no longer matches an action button, so the green rule can apply, assuming no other competing declarations change the result. This is only a possible fix: check the actual winning rule and intended styles before editing. If you cannot change the stylesheet that causes the conflict, you may need a different, deliberate strategy.

It can be reasonable when an important declaration is genuinely needed and you cannot appropriately change the competing styles—for example, when integrating with a stylesheet maintained elsewhere. Keep the use narrow and leave a comment explaining why it is intentional. Avoid using it as a routine way to defeat a selector you have not investigated. MDN notes that important declarations change normal cascade behavior and can make conflicts harder to debug, particularly in large stylesheets. [S2]

The practical cost is that someone changing the ordinary rule later may see no effect and may respond by adding another important declaration. That creates competing exceptions instead of one clear source of truth. Important declarations also follow cascade rules that differ from ordinary declarations: origin and layer order are reversed for important declarations, while specificity and source order still matter for ties at the relevant origin and layer. [S1] So do not assume that “later” or “more specific” alone settles every important-rule conflict.

Use this sequence:

1. Identify the exact element and property that are wrong.
2. Find the winning declaration and the competing declarations; note their origins, layers, selectors, and order.
3. Check whether the conflict comes from a broad rule, an unintended selector match, or a stylesheet you can change.
4. Prefer correcting the source rule or intended selector over escalating to !important.
5. If an important declaration is necessary, make it as narrow as practical and comment on the reason.
6. Recheck the computed result and nearby elements so the fix does not create another unintended conflict.

Without seeing the page’s stylesheets and cascade conditions, it is not possible to determine which declaration wins in your specific case.