Had the same issue using WeTheme's Flow theme, but found a hacky workaround.
First, I removed the logo image from the template customizer. That causes the template to default to your site name in text where the logo would go. Then I used Chrome inspector to find class name for element where that text appears. for me it was called site-header__logo. Then in global settings, I uploaded a .svg version of my logo to the "Files" section and copied the image URL. Next went in to CSS, (for Flow theme it was called timber.scss.liquid) and there I added code to set the opacity to 0 for the element using the class I just grabbed from the inspector, which was .site-header__logo-link. This made the site name transparent and no longer visible on the page. Finally, I went back in the Chrome inspector and found the parent element for the previous element. It was .site-header__logo (good ol' BEM) and then back in CSS, added in my .svg file as a background image... Here's what my CSS looked like:
background-image: url("url for svg file goes here");