You can enable TextMeshPro support with Edit / Project Settings… / UI Themes / TextMeshPro Support / Enable. If TextMeshPro not installed option will not be available.
You can disable support the same way with Edit / Project Settings… / UI Themes / TextMeshPro Support / Disable.
Support is enabled only to installed platforms. Platforms that were added after it requires enabling support again.
TextMeshPro support is enabled by adding
UIWIDGETS_TMPRO_SUPPORT directive to the Scripting Define Symbols in the Player Settings and forced scripts recompilation.