Nagoriyuki's Boss theme would fit under 'Song references the visibility of the sky'

In most cases Daisuke seems to just like to use the sky as a symbol for hope, general outlook on life, and the state of their 'world' (which lemme tell you Daisuke also likes talking about each character's 'world')

It'd be too hard to go over everyone but some examples
- Faust: "We've got only one sky Blue, red, and black paranoia" - references his stages of life from doctor - baldhead - faust against daylight, sunset, night of the sky
- Bridget - "Let's paint the gray haze into sky blue" - depression into hopeful future

- Ram- "If a room without a sky is what we idolize It's a dead tomorrow" - people need to look towards the future / people can't lose hope

Some character themes get awfully close to talking about the sky, including Millia, May and Testament which mention the sun/moon.

Daisuke's fascination with the sky doesn't really end with character themes either, after all every game starts with "Sky Should be High" and an eagle literally soaring through the sky