Change color, text of Read More button

1/ To change color of Read More button
By default, Read More button of Content Views has green background, white text color. But this can be override by style of the active theme.

To change it, please add this CSS code to file style.css of your active theme:

.pt-cv-readmore {
    color: YOUR_COLOR !important;
    background-color: YOUR_COLOR !important;
}

2/ To change text of Read More button
Since Content Views 1.8.8, you can change text of Read More button under:
Display Settings >> Fields Settings >> Content

Best regards,