Hi,
Please check whether the following is helpful:
Step 1. Click on Page Layout tab.
Click on Colors drop-down > at the bottom, click on Customize Colors... option.
Step 2. A Create New Theme Colors dialog box will open.
Please refer to the following screenshot for instructions.
Result: for this workbook, the default font color is changed to red.
Useful link: Change a theme and make it the default in Word or Excel.
Please respond if You require assistance. I will be glad to be of help.
If I was able to help You, please mark My response as answer and helpful.
Thank You!