Find a way to include the rule you want after the rule you're trying to override.
CSS are, by definition, Cascading Style Sheets. That means that the following code:
.divwhatever { background-color: black; } // On WordPress Original File
.divwhatever { background-color: white; } // On WordPress Theme File
.divwhatever { background-color: red !important; } // On WordPress Child Theme
.divwhatever { background-color: orange ! important; } // On a specific File
In the end will be presented with a Orange background.
The last processed rule with always be applied unless there's a previous !important
rule.
In that case, the last processed rule with the !important
modifier will prevail.
That's a very bad practice: ideally you should find the original rule and change it or, at the very least, have no more than one !important
statement for the same DOM model but I know how tough WordPress Themes can be to a beginner :)