Accessibility

When I check my site with Wave Web accessibility Chrome extension, it says the search form button is empty or has no value text.
Why It Matters
When navigating to a button, the descriptive text must be presented to screen reader users to indicate the function of the button.
How to Fix It
Place text content within the <button> element or give the <input> element a value attribute

I am not a coder at all, so can you help with supplying some sitewide CSS to fix.
Thanks