The phrase "a substantial theme" is correct and usable in written English.
It can be used when discussing a significant or important idea or topic within a piece of writing, art, or conversation.
Example: "The novel explores a substantial theme of redemption and forgiveness throughout its narrative."
Alternatives: "a significant theme" or "an important theme".
At 4 ½ minutes, it's a relatively substantial theme park experience, and the pace is varied enough that those prone to motion sickness should be able to handle it.
It is likely, in fact, to be the far more substantial theme here, as there are more efficient ways to flagrantly and gratuitously display opulence and excess, usually involving something tangible, like a personal resort flotilla mounted on a military destroyer frame.
A book needs a more substantial and substantiated theme than this.
"Serenity" throws together, in an unconsidered way, some other substantial themes, including the American way of violence, the transmission of male violence, the inability of American society to protect women and children from domestic abusers, the societal and personal exactions of an unjustified war, and the earnest role of popular digital entertainment in creating a self-image.
Lowell took on substantial themes and envisioned himself as a tragic, heroic figure, fighting against his own demons.Mr Ashbery's verse, by contrast, is more beguilingly casual.
A subtle writer, he demands an attentive reader as he explores, through superficially familiar narratives, substantial themes.
In the follow-up interviews conducted in 2008, the fears of discrimination and the difficulties in negotiating disclosure and sustaining intimate sexual relationships remained substantial themes.
By the next paragraph of a substantial article, the real theme was introduced, how, "Once again, TV finds itself in another cultural hot zone".
A Capstone Course may allow you to attain an extraordinarily high degree of skill in your area of focus, or to complete a substantial project on a theme or issue that you find deeply significant.
