Examiners read many theses. Clear, consistent figures make yours easier to follow and look more professional.
Use the same font, colours and style in every chapter. If control is grey in chapter 3, keep it grey in chapter 5.
Each figure should answer one question. If a figure needs a long explanation, split it into panels.
Low-resolution images copied from papers look poor in print and can raise copyright problems. Redraw schematics yourself, or describe them to a figure tool and cite the original source where needed.
Say what error bars show, how many replicates there were, and which test was used.
Spelling mistakes in figures are very common. Read every label aloud before submitting.
With SciFigures you can photograph your rough sketches and get clean figures back in one consistent style, then refine them with simple instructions.