Hi there,
currently there are several options but they all require a little bit of code. The next version of GB and the future GB pro will make this easy.
I would do the following:
1. In the left Grid column. Add a background image. Give the column a Settings > Advanced > Additonal CSS class of has-svg
2. Inside the left grid column add a HTML widget and add this code:
<div class="svg-container">
<svg width="100%" height="100%" viewBox="0 0 100 100" preserveAspectRatio="none">
<polygon points="80,100 100,0 100,100"></polygon>
</svg>
</div>
3. Add this CSS:
.has-svg {
position: relative;
}
.svg-container svg {
position: absolute;
top: 0;
right: 0;
bottom: 0;
left: 0;
fill: #f00; /* Set color here */
}