@DungLT,
Change of theme is not much noticeable in blank worksheet. You can test it by creating an empty workbook with MS Excel and change the theme. I can not notice any problem using Aspose.Cells for .NET 19.10.4 version. Share screenshots if you still notice this issue.
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.
Good to know that your issue is sorted out by the suggested code segment. Feel free to contact us any time if you need further help or have some other issue or queries, we will be happy to assist you soon.