Here is a flat design circle button:
.btn {_x000D_
height: 80px;_x000D_
line-height: 80px; _x000D_
width: 80px; _x000D_
font-size: 2em;_x000D_
font-weight: bold;_x000D_
border-radius: 50%;_x000D_
background-color: #4CAF50;_x000D_
color: white;_x000D_
text-align: center;_x000D_
cursor: pointer;_x000D_
}
_x000D_
<div class="btn">+</div>
_x000D_
but the problem is that the +
might not be perfectly centered vertically in all browsers / platforms, because of font differences... see also this question (and its answer): Vertical alignement of span inside a div when the font-size is big