I question what happens when you have a longer text that 'A' and the 1st dimension row is only 1 row high (how will it fit?) However, if that's not the case in your scenario, this should do it:
.sapzencrosstab-HeaderRow .sapzencrosstab-HeaderCellFirstInRow .sapzencrosstab-HeaderCellContentDiv { color : #009966; transform: rotate(-90deg); }
(The color : #009966 rule is just to call out what's changing in screenshot below):
