Manage landing page (refresh if slider not working thank you)

Please refresh if slider is not working :D

I used over 700 lines of css which feels a bit much.

The slider was my particularly difficult, as this is my first time, and I couldn't do it a concise way. Might do another project and improve it next time around. I have already spent a week on this issue and its the best I could manage for now.

Form validation was also very tricky, and i could not replace the default error message.

I am wondering how to improve on all those.