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typedef struct {
    char[4] magic;                      // File format magic number, "JHMF" in ASCII encoding
    int32 version;                      // File format version (currrently 122)
    int32 ep_count;                     // Number of recent export paths
    p_char[ep_count] export_paths;      // Recent export paths
    bg_image[3] bg_images;              // Background images
    int32 group_count;                  // Number of groups in the map
    Group[group_count] groups;          // Group objects
    int32 visgroup_count;               // Number of VisGroups
    VisGroup[visgroup_count] visgroups; // VisGroups objects
    Vector cordon_min;                  // Minimum corner of the Cordon box
    Vector cordon_max;                  // Maximum corner of the Cordon box
    int32 camera_count;                 // Number of Camera objects
    Camera[camera_count] cameras;       // Camera objects
    int32 path_count;                   // Number of path objects
    Path[path_count] paths;             // Path objects (created with the Path tool)
    Entity[] entities;                  // All entities including worldspawn, read until the end of the file
} Jmf;

typedef struct {
    p_char path;                        // Background image path
    double scale;                       // Image scale
    int32 luminance;                    // Image brightness (0-255)
    int32 filtering;                    // Image filtering (0=nearest, 1=linear)
    int32 invert;                       // Invert image colours
    int32 offset_x;                     // Image horizontal offset from center
    int32 offset_y;                     // Image vertical offset from center
    byte[4] unknown;
} bg_image

! this keeps the snowflakes white when using Dark Reader
twhl.info##.header-desktop .snowfield .snowflake:style(background:#fff !important)

The disadvantages are as follows:
You cannot edit world brushes whatsoever.
You cannot do crazy texture scaling since it's tied to the lightmap size.
You cannot edit clip nodes.

I don't suppose you've considered providing the same skybox at different times of day?