MagicPattern has an amazing collection of beautiful pure CSS background pattern to spice up your designs. I highly recommend checking out the full collection. Meantime, here are my top picks to get you started:

Stripes pattern

.lines-2 {
  background-color: #fefefe;
  background-image: linear-gradient(
    to right,
    #5394fd,
    #5394fd 12px,
    #fefefe 12px,
    #fefefe
  );
  background-size: 24px 100%;
}

Zig zag pattern

.zigzag {
  background-color: #fefefe;
  background-image: linear-gradient(135deg, #5394fd 25%, transparent 25%),
    linear-gradient(225deg, #5394fd 25%, transparent 25%),
    linear-gradient(45deg, #5394fd 25%, transparent 25%),
    linear-gradient(315deg, #5394fd 25%, #fefefe 25%);
  background-position:
    24px 0,
    24px 0,
    0 0,
    0 0;
  background-size: 48px 48px;
  background-repeat: repeat;
}

Polka dot pattern

.polka {
  background-color: #fefefe;
  background-image: radial-gradient(#5394fd 1.2px, #fefefe 1.2px);
  background-size: 24px 24px;
}

Checkerboard pattern

.rectangles {
  background-color: #fefefe;
  background-image: repeating-linear-gradient(
      45deg,
      #5394fd 25%,
      transparent 25%,
      transparent 75%,
      #5394fd 75%,
      #5394fd
    ),
    repeating-linear-gradient(
      45deg,
      #5394fd 25%,
      #fefefe 25%,
      #fefefe 75%,
      #5394fd 75%,
      #5394fd
    );
  background-position:
    0 0,
    24px 24px;
  background-size: 48px 48px;
}

Rhombus pattern

.rhombus {
  background-color: #fefefe;
  background-image: linear-gradient(135deg, #5394fd 25%, transparent 25%),
    linear-gradient(225deg, #5394fd 25%, transparent 25%),
    linear-gradient(45deg, #5394fd 25%, transparent 25%),
    linear-gradient(315deg, #5394fd 25%, #fefefe 25%);
  background-position:
    24px 0,
    24px 0,
    0 0,
    0 0;
  background-size: 24px 24px;
  background-repeat: repeat;
}

Paper pattern

.paper {
  background-color: #fefefe;
  background-image: linear-gradient(#5394fd 4.8px, transparent 4.8px),
    linear-gradient(90deg, #5394fd 4.8px, transparent 4.8px),
    linear-gradient(#5394fd 2.4px, transparent 2.4px),
    linear-gradient(90deg, #5394fd 2.4px, #fefefe 2.4px);
  background-size:
    120px 120px,
    120px 120px,
    24px 24px,
    24px 24px;
  background-position:
    -4.8px -4.8px,
    -4.8px -4.8px,
    -2.4px -2.4px,
    -2.4px -2.4px;
}

Diagonal pattern

.diagonal {
  background-color: #fefefe;
  background: repeating-linear-gradient(
    45deg,
    #5394fd,
    #5394fd 12px,
    #fefefe 12px,
    #fefefe 60px
  );
}

Triangles pattern

.triangle {
  background-color: #fefefe;
  background-image: linear-gradient(45deg, #5394fd 50%, #fefefe 50%);
  background-size: 24px 24px;
}

Waves pattern

.wavy {
  background-color: #fefefe;
  background-image: repeating-radial-gradient(
      circle at 0 0,
      transparent 0,
      #fefefe 24px
    ),
    repeating-linear-gradient(#5394fd55, #5394fd);
}

Isometric pattern

.isometric {
  background-color: #fefefe;
  background-image: linear-gradient(
      30deg,
      #5394fd 12%,
      transparent 12.5%,
      transparent 87%,
      #5394fd 87.5%,
      #5394fd
    ),
    linear-gradient(
      150deg,
      #5394fd 12%,
      transparent 12.5%,
      transparent 87%,
      #5394fd 87.5%,
      #5394fd
    ),
    linear-gradient(
      30deg,
      #5394fd 12%,
      transparent 12.5%,
      transparent 87%,
      #5394fd 87.5%,
      #5394fd
    ),
    linear-gradient(
      150deg,
      #5394fd 12%,
      transparent 12.5%,
      transparent 87%,
      #5394fd 87.5%,
      #5394fd
    ),
    linear-gradient(
      60deg,
      #5394fd77 25%,
      transparent 25.5%,
      transparent 75%,
      #5394fd77 75%,
      #5394fd77
    ),
    linear-gradient(
      60deg,
      #5394fd77 25%,
      transparent 25.5%,
      transparent 75%,
      #5394fd77 75%,
      #5394fd77
    );
  background-size: 48px 84px;
  background-position:
    0 0,
    0 0,
    24px 42px,
    24px 42px,
    0 0,
    24px 42px;
}