The standard file upload button is a regular looking button with the text Choose File.
You can easily fix this by using a label FOR the upload button and hiding the upload button. You then use css code to make the text look any way you want.
Because the label for attritube forwards the click to the corresponding element with the id that is the same as the for, clicking on the label text clicks on the choose file button. Simply style the label text to your hearts content.
You then would use Javascript code to process the file that is selected and of course make sure you validate/santize the file with the appropriate php code.