Changes to CSS not showing up on 404 page

Changes made to CSS file do not get applied on 404 error page

EDIT: page in question: https://hinnahackers.no/404/

Right of the bat: This is not a relative path issue. I am specifying a base href in my header like so:

<base href="https://hinnahackers.no/">

I know that this works, because my 404 page loads SOME of the styles in my CSS file, but not newly added classes.

It's also not a force refresh CSS file issue, because I am already doing styles.css?v=X:

<link href="css/styles.css?v=749" rel="stylesheet" type="text/css" />

But it doesn't seem to work on my 404 page. The CSS updates on other pages though, like index.php.

This is my .htaccess file that I use to redirect users to 404.php if they enter an invalid url:

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^([^/]+)/$ $1.php
RewriteRule ^([^/]+)/([^/]+)/$ /$1/$2.php
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !(\.[a-zA-Z0-9]{1,5}|/)$
RewriteRule (.*)$ /$1/ [R=301,L]

#Rewrite everything to https
RewriteCond %{HTTPS} !=on
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

#404 redirect
ErrorDocument 404 /404.php

In my CSS I added the following classes:

.404__heading {
    font-size: 2em;
    color: #fff;
    line-height: 1.5;
}

.404__error {
    margin-top: 50px;
    font-size: 2em;
    color: #ea0548;
    line-height: 1.5;
}

.404__entry__container {
    margin-top: 50px;
}

.404__entry__wrap {
    position: relative;
    display: inline-block;
    height: 20px;
    width: 21px;
    vertical-align: middle;
    margin-right: 12px;
}

.404__entry__wrap a {
    display: inline-block;
    vertical-align: middle;
    color: #fff;
    font-size: 2em;
    text-decoration: none;
}

.404__entry__top__dot {
    position: absolute;
    top: 0;
    left: 0;
    background: #ff6600;
    height: 7px;
    width: 7px;
}

.404__entry__middle__dot {
    position: absolute;
    top: 10px;
    left: 7px;
    background: #ff6600;
    height: 6px;
    width: 7px;
    transform: translateY(-50%);
}

.404__entry__bottom__dot {
    position: absolute;
    bottom: 0;
    left: 0;
    background: #ff6600;
    height: 7px;
    width: 7px;
}

.404__entry__caret {
    display: inline-block;
    vertical-align: middle;
    background: #fff;
    height: 35px;
    width: 18px;
    margin-bottom: -5px;
}

.404__heading::selection,
.404__error::selection,
.404__entry__wrap::selection,
.404__entry__wrap a::selection {
    text-shadow: none;
    background: rgba(255, 255, 255, 0.3);
    color: #fff;
}

But they don't get recognised on my 404.php page. So right now I have added styling etc. directly in my 404 page. I am completely at loss at what is going on. I have been googling for hours, but can't find a solution.