Reid Havens has a few tips for making Power BI reports look nicer:
This is less of a single applied step as it is multiple formatting practices applied throughout the report. I’ve already hit on this subject a little bit in the two previous Power BI visual design practices in regards to using complimentary colors. The two key takeaways in this section are object formatting and color coordination.
Of all my best practices I’m showcasing here I’d say this one is the most subjective. However I think that maintaining complimentary colors goes a long ways to creating a professional looking report. I also have a strong dislike for the default title design for visualizations in Power BI. By default it is left aligned and a grey color (AGAIN…hard to read!). I center that sucker and color the background. An added benefit to coloring the title background is it actually forces me to make sure my objects are aligned, otherwise it is VERY noticeable now if they aren’t.
Definitely read the comments on this one, as some of these tips are subjective.