Hi,
We need to enable/disable themes from site admin of wordpress MU so that active themes can be used by the bloggers. Please check these steps to accomplish it:
1) Login into wordpress MU site admin.
2) Click on themes from site admin section. [Check Image].

3) Select the themes which we needs to be activated for bloggers. [Check Image].

4) Click on update option to apply changes.
That’s it!
Thanks,
Shane G.
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