Say: "Who is Lord of the seven heavens, and Lord of the Great Throne!"
means, "Who is the Creator of the higher realm with its planets, lights and angels who submit to Him in all regions and in all directions! Who is the Lord of the Great Throne, which is the highest of all created things!"
Allah says here:
...
وَرَبُّ الْعَرْشِ الْعَظِيمِ
and Lord of the Great Throne,
meaning the Mighty Throne.
At the end of the Surah, Allah says:
...
رَبُّ الْعَرْشِ الْكَرِيمِ
the Lord of the Supreme Throne! (23:116),
meaning splendid and magnificent.
The Throne combines the features of height and vastness with splendor and magnificence. This is why it was said that it is made of red rubies.
Ibn Mas`ud said,
"There is no night or day with your Lord, and the light of the Throne is from the Light of His Face."
سَيَقُولُونَ لِلَّهِ قُلْ أَفَلَ تَتَّقُونَ