The phrase "a theme which is" is correct and usable in written English.
It can be used when introducing or discussing a specific theme in a text, analysis, or conversation.
Example: "The novel explores a theme which is prevalent in many coming-of-age stories."
Alternatives: "a theme that is" or "a theme which exists".
Many newspaper features will have a theme which is usually the basis for the story.
It is a theme which is beginning to emerge from the Bush administration.
The human world seen through the eyes of an animal is a theme which is almost comic in itself.
Each issue is based on a theme which is then shared and discussed, and ideas are generated for stories.
Some fear that a debate on the euro would strengthen the FN and encourage further demands for a return to national sovereignty, a theme which is increasingly popular across various categories of voters.
Mr. Giuliani did not cite values as a theme, which is unusual for a major Republican presidential candidate, and the omission seemed especially striking Monday, coming as it did just hours after a report that he and his former wife had donated $900 to an abortion rights advocacy group and provider.
Mr. Nixon's campaign had a theme — which was to bring an end to the war and disorder of the Johnson years — but it lacked any clue to how he proposed to carry it out.
It was my most difficult book – the book that treated a theme which was so distant, so remote, from my own emotional life that it gave me a special pleasure to use my combinational talent to make it real".
"It was my most difficult book," he said, "the book that treated a theme which was so distant, so remote, from my own emotional life that it gave me a special pleasure to use my combinational talent to make it real".
Nine would-be actors/actresses, one writer/observer, not knowing what the result of their efforts would be other than a theme, which was creating a ceremony for the nuclear age.
For example a participant stated, "good technical decisions can be reversed by political forces", a theme which was common from all three stakeholder workshops.
