Hi Quaerkrad, you can't seal ceramics as they would not take the sealer (Only Natural stone and the like require sealing)
To keep the grout good you could apply something like this
GROUT PROTECTOR 500ML
Price: £14.99
Quantity
What it’s for
Protecting all grout joints on wall and floor tiles
Advantages
Protects the grouts in ceramic tile floors and walls
Prevents the absorption of stains, particularly greasy ones
Eases the removal of dirt
Water based
Does not alter the surfaces appearance or colour