After further evaluation, I am able to observe the issue as you mentioned by creating a new workbook (file) by Aspose.Cells APIs. I found that Theme and Color are not changed for the output file (by Aspose.Cells) in MS Excel. I opened the output file into MS Excel manually, clicked on Page Setup and then clicked on Themes. I noticed when hovering over different themes (by mouse) in the list does not change the row/column matrix (UI). Even I changed to some other theme and it does not change anything. I then tested by opening a new Workbook in Ms Excel manually and did the same procedure which works ok. I have logged a ticket with an id “CELLSNET-46993” for your issue. We will look into it soon.
Once we have an update on it, we will let you know.