FilePicker: Adapt "+" Button styling to the one from the default file listing

[AndyScherzinger]

See nextcloud/server#37207 for reference (align with @szaimen - since the change needs to be aligned, if referenced ticket can't make it for 27.1 then go for the current styling (grey background)

Current - Files:

Current - Picker

Proposed Files:

[szaimen]

For my part:

<a href="#" class="button new" aria-expanded="false" aria-label="New file/folder menu">
	<span class="icon icon-add"></span>
	<span>New</span>
</a>

.app-files .actions .button.new {
    position: relative;
    width: unset;
    gap: 5px;
    background-color: var(--color-primary-element-light);
    color: var(--color-primary-element-light-text);
    border: var(--color-border-maxcontrast) 1px solid;
}

[szaimen]

My PR in nextcloud/server#39884

[susnux]

FilePicker PR: #892