Hello Amanda,
You can use this code:
/* Change the background color */
.bctt-click-to-tweet {
background-color: #BBEB2A;
}
/* Change the color of the text */
.bctt-ctt-text a {
color: #ff0000!important;
}
/* Change the color of the Click to tweet */
a.bctt-ctt-btn {
color: #0051ff!important;
}
I added some code to change the color of the texts too, just change the hex values and you are good to go.
If you need help to use this code, check this tutorial: https://generatepress.com/knowledgebase/adding-css/
If you need help with the color values, check this: http://www.color-hex.com/