Upload Button doesn't show properly on FF and IE

Styling for upload buttons seems to be based largely on browser defaults for input[type=”file”].

Current theme style sheet at https://www.digitaljobs.fr/wp-content/themes/digital-jobs/style-twentytwelve.css includes only following very minimal styling:

input[type="checkbox"], input[type="radio"], input[type="file"], input[type="hidden"], input[type="image"], input[type="color"] {
    border: 0 none;
    border-radius: 0 0 0 0;
    padding: 0;

Found following links:
https://stackoverflow.com/questions/572768/styling-an-input-type-file-button
https://stackoverflow.com/questions/12094321/how-would-one-go-about-styling-the-input-type-file-button

Appears you can’t change through CSS as you would with other elements. Workarounds do exist, but none of them are completely satisfactory. You need to use a JavaScript overlay that hides the display of the File Upload, see – https://www.quirksmode.org/dom/inputfile.html