fix(file-input): make all elements styleable with CSS variables + fix Bootstrap invalid box-shadow

[adrianptrv]

Closes #1676

This PR is related to the fix: #439

I have added CSS variables for parts of the file-input that couldn't be styled individually before, using only variables.
I also fixed the box-shadow of the Bootstrap invalid state on the file-input, input, select, and textarea components so that it only appears when the component is focused, and not all the time.

[desig9stein]

When I change the filename background, the input looks like this.

Is this the expected behavior?

To change the entire background of the file input, I need to modify the box-background variable, which is part of the input-group, to match the file-name-background, which is part of the file type.

[simeonoff]

Why are changes being made to the select and textearea themes in this PR?

[simeonoff]

Shouldn't this come from the $theme? I see this repeated in all files and it doesn't sit right.

[simeonoff]

When I change the filename background, the input looks like this.

Is this the expected behavior?

To change the entire background of the file input, I need to modify the box-background variable, which is part of the input-group, to match the file-name-background, which is part of the file type.

You are changing the file names background so it looks correct to me.

[simeonoff]

@desig9stein Can you verify the PR please?