you can override other styles by adding !important
to the style
textarea.v-textarea {
font-family: 'Lucida Console', Monaco, monospace;
background-color:black !important;
color: light green;
}
I would also search all the css and see if somewhere there is something like this and remove it
textarea.v-textarea::selection {
font-family: 'Lucida Console', Monaco, monospace;
background-color:white;
color: light green;
}