selector

Hi,

I'm new to CSS and have a hard time finding the correct selectors and classes.

On this blogpost: https://www.a-c-d.net/colorful-space-cowboy-illustrations-by-dalek2020/

I have a wordpress theme and use gutenberg image gallery (wordpress).

I want to add a space between each image except the last one.

I gave the gallery Additional CSS Class: space_gallery

I tried different classes and selectors without luck.

Thanks

Ben

Given the following code :

<section id="ourteam">
    <div class="container">
        <div class="margin-top-50"></div>
            <div class="row">
                    <div class="col-md-12 text-center">
                            <h1 class="banner h-ourteam">Meet Our Team</h1>
                    </div>
            </div> <!-- end of row -->
    </div> <!-- end container -->        
            <div class="margin-bottom-50"></div>
                 <div class="container">
   <div class="well">
            <div class="margin-top-50"></div>
    <div class="row">

I would like to insert some CSS code on a Wordpress website to override a height value on WP Flow Plus. Looking in Chrome's Inspect Element I can see the HTML for what I want to change:

<div id="wpif2_flowplus_1" class="wpif2_flowplus" style="color: rgb(85, 85, 85); width: 900px; height: 750px; background-color: rgb(255, 255, 255);">

I want to change the height to 530px so I am looking for the correct CSS syntax. I have tried

#wpif2_flowplus_1 {
height: 530px;
}

But nothing happens. Changing it in Dev tools works.

I'm very confused, because :hover selector was working normaly, but :active not working.
The case is when click $name the page script was same but different URL because using ?room=<?php echo $ID;?> and when hover name occurred color blue and after click occurred still blue (active). In this problem, hover working normally but selector active not working (

The code:

::CSS::
#working
selec:hover{background-color:blue;}

#not work
selec:active{background-color:red;}

::PHP::

I've read alot online about this, but for some reason I just cannot understand this, so if someone could please explain in plan words I would sure appriciate it.

I thought that ex. #nav ul li this one goes for ALL <li> that are children og <ul> that are child of #nav.

But what is the difference between that one and this one : #nav > ul > li ?

If someone could please explain this ( preferbly with a very simple html explanation that would be GREAT