To override the primary colour setting on individual forms, you can place the following custom <style> code on your form’s Advanced Tab in the Code head block:

<style>
  /* Change --primary-color to customize the base colour for links and buttons, etc */
  /* Change --primary-contrast-color to black if your primary colour is very light, so you can read the text on buttons */
  :root {
    --primary-color: black;
    --primary-contrast-color: white;
    
    /* Leave everything below this line as is */
    --link-hover-color: color-mix(in srgb, var(--primary-color) 80%, white 20%);
    --btn-hover-bg-color: color-mix(in srgb, var(--primary-color) 85%, white 15%);
    --btn-hover-border-color: color-mix(in srgb, var(--primary-color) 90%, white 10%);
    --btn-focus-bg-color: color-mix(in srgb, var(--primary-color) 85%, white 15%);
    --btn-focus-border-color: color-mix(in srgb, var(--primary-color) 90%, white 10%);
    --btn-active-bg-color: color-mix(in srgb, var(--primary-color) 80%, white 20%);
    --btn-active-border-color: color-mix(in srgb, var(--primary-color) 90%, white 10%);
    --input-focus-border-color: color-mix(in srgb, var(--primary-color) 50%, white 50%);
    --primary-rgba-25: color-mix(in srgb, var(--primary-color) 25%, transparent 75%);
    --btn-focus-rgba-50: color-mix(in srgb, var(--btn-focus-bg-color) 50%, transparent 50%);
  }

  /* Fallback for browsers that don't support color-mix() (mainly Safari < 16.4) */
  @supports not (color: color-mix(in srgb, red, blue)) {
    :root {
      --link-hover-color: hsl(from var(--primary-color) h s calc(max(l + 20, 20)));
      --btn-hover-bg-color: hsl(from var(--primary-color) h s calc(max(l + 15, 15)));
      --btn-hover-border-color: hsl(from var(--primary-color) h s calc(max(l + 10, 10)));
      --btn-focus-bg-color: hsl(from var(--primary-color) h s calc(max(l + 15, 15)));
      --btn-focus-border-color: hsl(from var(--primary-color) h s calc(max(l + 10, 10)));
      --btn-active-bg-color: hsl(from var(--primary-color) h s calc(max(l + 20, 20)));
      --btn-active-border-color: hsl(from var(--primary-color) h s calc(max(l + 10, 10)));
      --input-focus-border-color: hsl(from var(--primary-color) h calc(s * 0.6) calc(max(l + 30, 50)));
      --primary-rgba-25: hsl(from var(--primary-color) h s l / 0.25);
      --btn-focus-rgba-50: hsl(from var(--primary-color) h s calc(max(l + 15, 15)) / 0.5);
    }
  }

  /* Ultra-fallback for browsers without relative color syntax (IE, very old browsers) */
  @supports not (color: hsl(from red h s l)) {
    :root {
      --link-hover-color: var(--primary-color);
      --btn-hover-bg-color: var(--primary-color);
      --btn-hover-border-color: var(--primary-color);
      --btn-focus-bg-color: var(--primary-color);
      --btn-focus-border-color: var(--primary-color);
      --btn-active-bg-color: var(--primary-color);
      --btn-active-border-color: var(--primary-color);
      --input-focus-border-color: var(--primary-color);
      --primary-rgba-25: var(--primary-color);
      --btn-focus-rgba-50: var(--primary-color);
    }
    
    /* Apply brightness filters to simulate lightening for ultra-old browsers */
    a:hover { filter: brightness(1.2); }
    .btn-primary:hover { filter: brightness(1.15); }
    .btn-primary:focus { filter: brightness(1.15); }
    .btn-primary:active { filter: brightness(1.2); }
    .btn-outline-primary:hover { filter: brightness(1.15); }
    .btn-outline-primary:active { filter: brightness(1.2); }
    .form-control:focus { filter: brightness(1.5); }
    .form-check-input:focus { filter: brightness(1.5); }
    .form-select:focus { filter: brightness(1.5); }
  }

  a {
    color: var(--primary-color);
  }

  a:hover {
    color: var(--link-hover-color);
  }

  .text-primary {
    color: var(--primary-color) !important;
  }

  .bg-primary {
    background-color: var(--primary-color) !important;
  }

  .btn-primary {
    background-color: var(--primary-color);
    border-color: var(--primary-color);
    color: #fff;
  }

  .btn-primary:hover {
    background-color: var(--btn-hover-bg-color);
    border-color: var(--btn-hover-border-color);
  }

  .btn-primary:focus {
    background-color: var(--btn-focus-bg-color);
    border-color: var(--btn-focus-border-color);
    box-shadow: 0 0 0 .25rem var(--btn-focus-rgba-50);
  }

  .btn-primary:active {
    background-color: var(--btn-active-bg-color);
    border-color: var(--btn-active-border-color);
  }

  .btn-primary:active:focus {
    box-shadow: 0 0 0 .25rem var(--btn-focus-rgba-50);
  }

  .btn-primary:disabled {
    background-color: var(--primary-color);
    border-color: var(--primary-color);
  }

  .btn-outline-primary {
    border-color: var(--primary-color);
    color: var(--primary-color);
  }

  .btn-outline-primary:hover {
    background-color: var(--btn-hover-bg-color);
    border-color: var(--btn-hover-border-color);
    color: var(--primary-contrast-color);
  }

  .btn-outline-primary:focus {
    box-shadow: 0 0 0 .25rem var(--btn-focus-rgba-50);
  }

  .btn-outline-primary:active {
    background-color: var(--btn-active-bg-color);
    border-color: var(--btn-active-border-color);
    color: var(--primary-contrast-color);
  }

  .btn-outline-primary:active:focus {
    box-shadow: 0 0 0 .25rem var(--btn-focus-rgba-50);
  }

  .btn-outline-primary:disabled {
    background-color: transparent;
    color: var(--primary-color);
  }

  .btn-check:focus + .btn-primary-activated,
  .btn-primary-activated:focus {
    box-shadow: 0 0 0 .25rem var(--btn-focus-rgba-50);
  }

  .btn-check:checked + .btn-primary-activated,
  .btn-check:active + .btn-primary-activated,
  .btn-primary-activated:active,
  .btn-primary-activated.active {
    background-color: var(--primary-color);
    border-color: var(--primary-color);
  }

  .btn-check:checked + .btn-primary-activated.active-hover:hover,
  .btn-primary-activated.active-hover:active:hover {
    background-color: var(--btn-active-bg-color);
    border-color: var(--btn-active-border-color);
  }

  .form-control:focus {
    border-color: var(--input-focus-border-color);
    box-shadow: 0 0 0 .25rem var(--primary-rgba-25);
  }

  .form-check-input:focus {
    border-color: var(--input-focus-border-color);
    box-shadow: 0 0 0 .25rem var(--primary-rgba-25);
  }

  .form-check-input:checked {
    background-color: var(--primary-color);
    border-color: var(--primary-color);
  }

  .form-select:focus {
    border-color: var(--input-focus-border-color);
    box-shadow: 0 0 0 .25rem var(--primary-rgba-25);
  }

  /* Ultra-fallback: Use opacity for shadow effects when RGBA isn't available */
  @supports not (color: hsl(from red h s l)) {
    .btn-primary:focus,
    .btn-outline-primary:focus,
    .btn-check:focus + .btn-primary-activated,
    .btn-primary-activated:focus {
      box-shadow: 0 0 0 .25rem var(--primary-color);
      box-shadow: 0 0 0 .25rem color-mix(in srgb, var(--primary-color) 50%, transparent);
    }
    
    .form-control:focus,
    .form-check-input:focus,
    .form-select:focus {
      box-shadow: 0 0 0 .25rem var(--primary-color);
      box-shadow: 0 0 0 .25rem color-mix(in srgb, var(--primary-color) 25%, transparent);
    }
  }

  .nav-tabs .nav-link.active {
    border-bottom-color: var(--primary-color);
  }
</style>