Hi Jaco,
At present, it is not feasible to customize the UI elements of the SfPdfViewer control using styles. The solution we shared earlier works for customizing the loading indicator that appears when the document is loading. But it is not working for the loading indicators that appear on each PDF page before it is rendered.
To achieve this requirement, we will provide support for themes. It will be available in any of our upcoming releases. We will let you know once this support is available.
Please find the feedback link below.
https://www.syncfusion.com/feedback/46158/customize-the-color-of-ui-elements-in-sfpdfviewer
Regards,
Salman