In order to change variables, go to the theme's CSS file. You'll see the following code at the very top of the file. Change values and click update. Now refresh and you'll see the updated changes. Note that the variables used are nearly identical to the ones used in my Sparox Ghost Theme.

/****** VARIABLES ******/
:root {
   --color-primary: var(--color-blue);
   --color-secondary: var(--color-yellow);
   --color-accent: var(--color-pink);

   /* Darkmode Colors */
   --color-bg-900: var(--color-black-900);
   --color-bg-700: var(--color-black-700);
   --color-bg-500: var(--color-black-500);
   --color-border: var(--color-dark-border);
   --color-text: var(--color-dark-text);
   --color-title: var(--color-dark-title);

   --font-size: 100%;
   --font-family-title: 'Barlow', sans-serif;
   --font-family-text: 'Rubik', sans-serif;

   --border-width: 1px;
   --border: var(--border-width) solid var(--color-border);
   --border-hover: var(--border-width) solid #414160;
   --border-glow: var(--border-width) solid rgba(255, 255, 255, .35);
   --border-glow-hover: var(--border-width) solid rgba(255, 255, 255, .5);
   --border-radius: .5rem;

   --transition: all .2s ease-in-out;

   --container-width: 1200px;

   --spacing-xs: .5rem;
   --spacing-s: 1rem;
   --spacing-m: 2rem;
   --spacing-l: 4rem;
   --spacing-grid: 1.5rem;
   --spacing-markup: 1rem;
   
   /* Colors */
   --color-blue: #17D2FF;
   --color-yellow: #EFC75E;
   --color-pink: #C850DE;
   --color-red: #BB2E41;
   --color-green: #4ADD71;

   /* Blacks */
   --color-black-900: #13131D;
   --color-black-700: #1A1A26;
   --color-black-500: #242431;

   /* Whites */
   --color-white-900: #F8F8FA;
   --color-white-700: #EEEEF4;
   --color-white-500: #E0E0E9;

   /* Darkmode & Lightmode */
   --color-dark-border: #32324A;
   --color-dark-text: #CFCCE0;
   --color-dark-title: #FFFFFF;
   --color-light-border: #D8D8E1;
   --color-light-text: #28282E;
   --color-light-title: #000000;
}

@media only screen and (max-width: 800px) {
   :root {
      --spacing-s: .75rem;
      --spacing-m: 1.5rem;
      --spacing-l: 3rem;
      --spacing-grid: 1rem;
   }
}