Add textarea#message to your CSS selectors:
@media only screen and (min-width: 768px) {
input[type=text],
input[type=email],
textarea#message {
width: 300px;
border: 1px solid blue;
}
}
I usually use my browser's developer tools (accessible via F12) to determine which classes and ids the custom rules should be applied to.
Just a quick reminder...
As you are using a lot of custom CSS in your theme's 'custom.css' file (which is absolutely fine), be sure to make a backup of this file.
If/when you next upgrade Showkase, your 'custom.css' file will be overwritten with the blank version that comes bundled within Showkase and you'll need to reinstate your custom version after upgrading (or you could remove the blank version from the Showkase core files before upgrading).