For languages that support #if and #ifdef statements, Source Insight recognizes inactive blocks of code based on the value of the conditions. You can edit the conditional values by selecting Options > Preferences: Language and clicking either "Project Specific Conditions" or "Global Conditions". You can also click inside a conditional identifier, then rigth-click and select Edit Condition.
Blocks of code that are inactive are given the "Inactive Code" style. For more information, see Conditional Parsing.