Hello,
1/ Use the next custom CSS code:
.single-product td.value select:hover {border-color: red;}
2/ Describe your request in more detail using screenshots, please.
3/ This text comes from the theme, so you can use translation files to change it. Install and activate Loco Translate plugin > create a translation for XStore theme > change English text and disable the Loco plugin https://xstore.helpscoutdocs.com/article/30-base-theme-translation
4/ Theme Options > Header builder > Header sticky
Regards