Sites that look thrown together usually are not badly designed so much as inconsistently designed: six blues, four heading sizes, and gaps that change from one section to the next. Kitset avoids that by giving you a small set of choices that apply everywhere.
You have a main colour, which does the work on buttons and links, a text colour and a background. Two fonts, one for headings and one for reading. Changing any of them updates every page at once.
The one thing worth taking seriously is the contrast warning. If the panel tells you a combination is too faint, that is not a matter of taste. It is the same measurement screen readers and Google use, and pale grey text on white is the most common way a nice-looking site becomes unreadable for somebody over fifty.
Step by step
In the editor, open the style panel.
Change a colour, a font, or the spacing.
Watch the page update as you go.
Take any contrast warning seriously before you publish.
If it does not go to plan
I want one thing a different colour, not everything.
Click just that thing and change it there instead. The style panel is for site-wide decisions.
How many colours should I use?
One strong one, plus your text and background. Sites with a single confident colour almost always look better than sites with four.