To begin customizing up your platform's color theme, click on the Settings button in the upper-right, then on Branding and Customization.
Click on the Theme Customization tab to go to the editor.
You will see several sections, with one for the Navigation Bar, Buttons, Section Headers & Tabs, Icons & Links, and Course Partitions. Just click on the color button to open the color picker.
On the color picker, you can click on the palette to select the color you want and drag the slider to change the selected hue. You can also directly enter the color information below the slider. Clicking on the default RGB (Red Green Blue) text will let you switch to HSL (Hue Saturation Luminance) and Hexcode modes, allowing you to precisely select which colors you will use.
The preview will be updated live, so you can see what your platform will look like before you commit to a theme. When you're satisfied, click the Update Branding button at the bottom of your screen.
If necessary, you can click on the Reset Customization button in the upper-right corner of the page to set your colors back to the default options.