You can adjust the colors of the candlesticks and the chart’s background by right-clicking on the chart and selecting the “Chart settings” option (or use the “Chart settings” button on the Charts tab).
By the way, you can create a custom template and make it default for all charts.
Make all necessary changes in the “Color Scheme”, “Settings”, and “All charts” tabs, and apply the changes.
Then save these changes as a template:
Right-click the chart window → Templates → Save template.
Give the following name to the template: “default”.
Click “Save” and overwrite the existing template.