Added styling to file selector button

[TheDro]

Closes #56

Purpose

Add styling to file inputs so that the "Choose File" button matches that of the other buttons in an app.

Result

Here are before and after pictures:

before

after

Notes

The border around the active button could use some improvement. Currently, it gets clipped by the outer input element.

I'm not sure whether this would merit a minor or patch version update. Let me know which is most appropriate and I'll make the update.

[TheDro]

Additional info: This change allows me to remove this "monkey patch" from a project I'm currently working on:

input[type=file]::file-selector-button {
    background-color: var(--primary);
    border: var(--primary);
    border-radius: var(--br);
    color: var(--white);
    padding: var(--sm-pad) var(--md-pad);
    display: inline-block;
    font-weight: 400;
    text-align: center;
    white-space: nowrap;
    vertical-align: middle;
    -webkit-user-select: none;
    -moz-user-select: none;
    user-select: none;
    border: 1px solid transparent;
    font-size: 1rem;
    line-height: 1.5;
    transition: color .15s ease-in-out, background-color .15s ease-in-out, border-color .15s ease-in-out, box-shadow .15s ease-in-out;
}
input[type=file]:hover::file-selector-button {
    background-color: var(--primary-600);
    border-color: var(--primary-600);
    color: var(--white);
}
input[type=file]:not(:disabled):active::file-selector-button {
    background-color: var(--primary-600);
    border-color: var(--primary-600);
    color: var(--white);
}
input[type=file]:focus::file-selector-button {
    outline: 0;
    box-shadow: 0 0 0 0.2rem rgba(59, 130, 246, 0.5);
}
input[type=file]:disabled::file-selector-button {
    opacity: .65;
    cursor: not-allowed;
    background-color: var(--primary);
    border-color: var(--primary);
    color: var(--white);
}

[TheDro]

Side note: I'd like to thank the author and maintainers of this project. It's really great and I've been using it for a bunch of my personal projects for years now.