Visibility

.blue-opacity-hover, .blue-opacity-hover-content-active, .blue-opacity-hover-content-default

With .blue-opacity-hover you can hide things by default and let them appear when the user hovers the area around it or focuses an element inside. This way you can simplify the UI a bit and make controls disappear when they are not needed. On touch screens the elements will always be visible. Usage:

<div class="card blue-opacity-hover p-3 flex-row justify-content-between">
    <h5 class="blue-opacity-hover-content-default">Hover here to see a button</h5>
    <input type="text" class="form-control me-1" placeholder="When you focus me, the button should appear too" />
    <button class="btn btn-secondary blue-opacity-hover-content-active">Hey there</button>
</div>

.blue-d-hover, .blue-d-hover-content-active, .blue-d-hover-content-default

.blue-d-hover is similar to .blue-opacity-hover, but the elements will disappear and appear with the display property. It is ideal if you want to replace something with something else when the user hovers. On the example I used it to change the edit and delete icons from lineout to filled when hovering. Usage:

<button class="btn btn-secondary blue-d-hover d-inline-flex align-items-center gap-1">
    <span class="blue-d-hover-content-default">⭕</span>
    <span class="blue-d-hover-content-active">🔵</span>
    Hover to fill the icon
</button>