First of all, as you said that you didn't even know that what you did was kufr/sin and now when you have realised that so it means you are now a changed person and won't do that again. Just ask for forgives from Allah that's it.
Allah is the (Rehman) Most Merciful of all. He likes the people who turn to Him in repentance.
Following are some quotes related to your question:
"Surely God loves those who turn much (to Him), and He loves those who purify themselves." — Quran, Sura 02 (Al-Baqara), ayah 222
"God accepts the repentance of those who do evil in ignorance and repent soon afterwards; to them will God turn in mercy: For God is full of knowledge and wisdom. Of no effect is the repentance of those who continue to do evil, until death faces one of them, and he says, "Now have I repented indeed;" nor of those who die rejecting Faith: for them have We prepared a punishment most grievous." — Quran, Sura 04 (An-Nisa), ayah 17–18
Prophet Muhammad (PBUH) said, "Every son of Adam sins, the best of the sinners are those who repent." — Sunan al-Tirmidhi, Hadith no. 2499
In Sahih al-Bukhari, Anas ibn Malik narrates: Allah's Apostle said, "Allah is more pleased with the repentance of His slave than anyone of you is pleased with finding his camel which he had lost in the desert." — Sahih al-Bukhari, 8:75:321.
There are plenty of Hadiths and Ayats emphasizing everyone to ask for forgiveness which clearly means that Allah (SWT) wants sinners to turn Him and ask for forgives and He is the only one who is the Most Gracious and the Most Merciful of all.