Add Sakai theme information to LTI launch data

[davidpbauer]

Add the theme base color set in sakai properties for light and dark mode as well as the user's dark mode preference to the LTI launch data. This is primarily used in Tsugi to match the Tsugi theme and Tsugi tool theme to whatever the theme is from the Sakai instance that launched the tool.

[davidpbauer]

@csev I'd love your opinion on this one. This is the custom we've added to Sakai (along with some changes in Tsugi) to send the Sakai theme base color and dark mode preference with every LTI launch. I know this is accomplishing something similar to your work with the custom Tsugi dark mode substitution values, but we are trying to avoid users having to understand that they need to include those custom values in the installation.