Hi @Samuel Stevenson,
I'm glad to hear you solve the problem ,if you have any issue about SharePoint, you are welcome to raise a ticket in this forum.
By the way, since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others." and according to the scenario introduced here: Answering your own questions on Microsoft Q&A, I would make a brief summary of this thread:
[Apply CSS to Remove Library Picker Dropdown in SPFX Extension]
Issue Symptom:
Need to apply custom css to remove some webpart in SharePoint modern page
Solution:
Inject custom CSS by following extension
SPFx Applications Customiser CSS Injection
Insert the following code to hide webpart in extension
button[title='Switch to other libraries on this site'] {
display: none !important;
}
You could click the "Accept Answer" button for this summary to close this thread, and this can make it easier for other community member's to see the useful information when reading this thread. Thanks for your understanding!