Is there a way to darken an engrave in the web app without going and modifying the SVG to upload?
The darkness of an engrave is purely a function of the material. Sometimes going deeper with more power or slowing the engrave will darken a line, sometimes not. More lines per inch often works. These can be changed by going into a manual engrave mode and changing settings. The width of a engraved line however can only be changed in a 3rd party editing package. For an engrave whatever the line width is going into the App is what you engrave. I doubt changing that within the app, if it were possible, would work very well.