In SharePoint Online it's possible to customize the look of your site via site settings under Look and Feel in the Change the Look link, and also change colors in the chosen theme.Themes are defined by spcolor files, and the association of a spcolor file to a particular theme can be located in site settings under Web Designer Galleries in the Composed Looks link.
A spcolor file is an XML file that has CSS values which allow users to change the look and feel of their site via SharePoint themes users interface.
There's a great SharePoint Color Palette Tool which helps you create a very detailed theme for SharePoint.
The following color names you will see in the Color Palette designer control the colors of Dashboard add-in part should you wish to change them:
EmphasisBackground – Main color of the header
EmphasisText – Text color in dashboard header
BodyText – Main add-in part text color
NavigationAccent – active tabs text color
Hyperlink – Links text color, Action buttons color
HyperlinkActive - Hyperlink color when pressed
SearchURL - Valid certificates color
ErrorText – Expired certificates color
Transcript uses same colors as dashboard add-in part.
Save your spcolor file after you've made the changes in the Color Palette Tool, and upload it to the themes Gallery (Site Settings > Web Designer Galleries >Themes > 15 folder). Now you can create a new Composed Look with it (Site Settings > Web Designer Galleries > Composed looks). You specify a Master Page URL, a Theme URL, an Image URL, and a Font Scheme URL, and that combination make up a Composed Look. Use your spcolor file in the Theme URL.
Once the new Composed Look is saved, you can apply it to the site in Site Settings > Look and Feel > Change the look.