For this, you will need to create your own Theme file or use a premade one. These can be found as an additional ZIP file in the latest release of Continuum.

First, enter Continuum's Content folder:

Here, you can paste or create an "XLTheme.txt" file. If valid, you can start Continuum and cycle through the UI Theme options until you reach a seventh option with a plain white icon. This will be your custom theme.

Additionally, a "theme.png" file can be added as a thumbnail.

Visit this page for information on how to create your own custom Themes.

These Themes are already in the downloadable Theme pack found with the latest release of Continuum Launcher (in order of creation):