you would need to change the image itself in order to change the color from "yellow" to something else, since the color is build into the image. it is not using CSS for the color.
Actually the border concerned is in style properties, go to appearance--->style properties -->building blocks---> primary content--> Look at border bottom.
The yellow gradient inside text field look under Forms--> text control, focused state --> clear the gradient bg image.
And maybe you should do also text area, focused state.