Use the live Customizer
Open Appearance → Customize. Every change previews instantly and nothing is published until you click Publish, so it is safe to experiment.
Keep a consistent palette
Pick one accent colour and use it for primary actions only. Too many accent colours make it harder for visitors to see what to click.
Edit page layouts
Pages are built with blocks. Open a page, click a block, and use the toolbar and the settings sidebar to change it. Reusable patterns live in the block inserter under Patterns.
Use a child theme for code changes
If you edit theme files directly, an update will overwrite your work. Put custom PHP and CSS in a child theme instead.