Outlines on blocks!

I have just started playing Minecraft and have already ran into a problem. Whenever I play I regularly see outlines on blocks that are the same color as the sky, blue outlines during day, orange outlines during sunrise/sunset, black outlines at night, yet the outlines aren't noticeable. Also, while I am in a cave or underground there seems to be no outlines at all. I am unsure if this is a bug or if it is my computer causing the problem, thank you for helping and have a great day!