I have never tried changing fonts or color but I've added the company logo on every sheet as it is required by many users.
I do not know if you familiar with this or not
To add an image (without any extension object)....and you can change alignment of your sheet name from left to right. I suggest keep on left only.
Click on any app you want to edit and once you open it there will be sheets below and on the top right side just below pen symbol you'll have a gear symbol.
Click on that gear symbol and you'll see options to add a sheet Image, Background color for sheet, font Color and Image Alignment.
Note: You should place your Image in C:\Users\username\Documents\Qlik\Sense\Content\Default
This is for Qlik Sense Desktop and same folder will be available on server as well...