You can choose built in themes that entail default styles for visual elements. Depending on the theme different elements will be changes such as the color of the plot background, margin, grid lines, axes color/thickness and even trace color.
The built-in themes are:
Plotly White Default setting
Plotly classic A clean look with clear dark axes.
Plotly gray Adds a light blue plot background.
Plotly Dark Both plot background and margin color are set black.
Ggplot2 Makes the plot background light grey and adds ticks to the y-ax.
These default themes you can use right away, however, at Theme > Create [link] you can create your own theme. These original themes are saved to your account and can be accessed through the Theme > Personal tab. Personal themes can also be shared between collaborators on a plot. You will be able to select the theme through Theme > Shared, and apply its styles to your plot.