Hacer el símbolo más en CSS

Solución:

Utilice varios antecedentes como a continuación:

.plus {
  display:inline-block;
  width:50px;
  height:50px;
  
  background:
    linear-gradient(#fff,#fff),
    linear-gradient(#fff,#fff),
    #000;
  background-position:center;
  background-size: 50% 2px,2px 50%; /*thickness = 2px, length = 50% (25px)*/
  background-repeat:no-repeat;
}

.alt {
  background:
    linear-gradient(#000,#000),
    linear-gradient(#000,#000);
  background-position:center;
  background-size: 50% 2px,2px 50%; /*thickness = 2px, length = 50% (25px)*/
  background-repeat:no-repeat;
}
.radius {
  border-radius:50%;
}

<div class="plus">
</div>

<div class="plus alt">
</div>

<div class="plus radius">
</div>

Y aquí está con transparencia:

.plus {
  width:50px;
  height:50px;
  display:inline-block;
  
  background:
    linear-gradient(#000,#000) top left,
    linear-gradient(#000,#000) top right,
    linear-gradient(#000,#000) bottom left,
    linear-gradient(#000,#000) bottom right;
  background-size: calc(50% - 1px) calc(50% - 1px); /*thickness = 2px (2*1px) */
  background-repeat:no-repeat;
  border:10px solid #000; /*length = 30px (50px - 2x10px) */
  box-sizing:border-box;
}

.radius {
  border-radius:50%;
}

body {
 background:pink;
}

<div class="plus">
</div>

<div class="plus radius">
</div>

Podemos agregar una variable CSS para controlar fácilmente la forma general:

.plus {
  --t:2px;   /* Thickness */
  --l:40px;  /* size of the symbol */
  --s:10px;  /* space around the symbol */
  --c1:#fff; /* Plus color*/
  --c2:#000; /* background color*/

  display:inline-block;
  width:var(--l);
  height:var(--l);
  padding:var(--s);
  box-sizing:border-box; /*Remove this if you don't want space to be included in the size*/
  
  background:
    linear-gradient(var(--c1),var(--c1)) content-box,
    linear-gradient(var(--c1),var(--c1)) content-box,
    var(--c2);
  background-position:center;
  background-size: 100% var(--t),var(--t) 100%;
  background-repeat:no-repeat;
}

.radius {
  border-radius:50%;
}

<div class="plus"></div>
<div class="plus" style="--l:35px;--t:3px;--c2:green"></div>
<div class="plus" style="--l:50px;--t:1px;--s:5px;--c1:red;"></div>
<div class="plus" style="--l:35px;--t:5px;--s:0px;--c1:blue;--c2:orange;"></div>

<br>
<div class="plus radius"></div>
<div class="plus radius" style="--l:35px;--t:3px;--c2:green"></div>
<div class="plus radius" style="--l:50px;--t:1px;--s:5px;--c1:red;"></div>
<div class="plus radius" style="--l:35px;--t:5px;--s:0px;--c1:blue;--c2:orange;"></div>

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