changed color of text and buttons to black

[huiningliang]

Because the background is a very light shade, changing the buttons to a dark color such as black should make it more obvious to users.

[huiningliang]

Resolved #82
Accessibility score has increased from 77 to 79; Colors of the instruction text as well as the cancel button are now changed to black, which is considerably more visible than the original light gray(?). The lighthouse score did improve, but not significantly, which is understandable given the size and difficulty of resolving this issue. The phrase "style = "color: black"" was added behind the lines related to the two recommended error locations, which changed the text color to black in both places.