Are viruses contagious through toilets?

Yes, any physical. Or airborne droplet contact may transmit viruses. Note this has been happening since the day you were born so most viral transmissions do not cause harm. Use common sense cleanliness - avoid direct skin contact on public toilet seats (or wipe with bleach towelette), wash hands thoroughly with soap and water, leave the place clean for the next user. At home use diluted bleach.