It works like this - whole json in single line with double quotes escaped by backslashes:
Code:
mmkv-cli set eac_app_md.obsidian "{\"activityConfigMap\":{},\"autoFreezeConfig\":{\"autoFreeze\":false,\"enable\":true,\"supportAutoFreeze\":true},\"autoStartConfig\":{\"autoStart\":false,\"enable\":true,\"supportAdjust\":true},\"colorConfigMap\":{},\"colorMode\":\"vivid\",\"cssConfigMap\":{},\"dpiConfig\":{\"dpi\":450,\"enable\":true},\"enable\":true,\"extraConfig\":{\"allowSplashScreen\":false,\"enable\":true,\"forceFullScreen\":false,\"fullPMAccess\":false,\"useDialogBorder\":false,\"usePageKeyAsVolumeKey\":true},\"forceScrollRefreshClsList\":[],\"globalActivityConfig\":{\"clsName\":\"\",\"disableScrollAnim\":false,\"displayConfig\":{\"bwMode\":0,\"cfaColorBrightness\":0,\"cfaColorSaturation\":50,\"cfaColorSaturationMin\":60,\"contrast\":30,\"ditherThreshold\":128,\"enable\":true,\"enhance\":true,\"monoLevel\":10},\"enable\":true,\"noteConfig\":{\"compatibleVersionCode\":0,\"drawViewKey\":\"com.getcapacitor.CapacitorWebView\",\"enable\":true,\"globalStrokeStyle\":{\"enable\":true,\"strokeColor\":-16777216,\"strokeExtraArgs\":[],\"strokeParams\":[],\"strokeStyle\":0,\"strokeWidth\":3},\"repaintLatency\":500,\"styleMap\":{},\"supportNoteConfig\":true},\"paintConfig\":{\"antiAlisingType\":0,\"ditherBitmap\":false,\"enable\":true,\"fillBrightness\":0,\"fillContrast\":0,\"fillEAC\":false,\"iconBrightness\":0,\"iconContrast\":0,\"iconEAC\":false,\"iconThreshold\":0,\"imgEAC\":true,\"imgGamma\":60,\"quantBits\":3,\"textBold\":false,\"textEACType\":0},\"refreshConfig\":{\"animationDuration\":50,\"antiFlicker\":10,\"enable\":true,\"gcInterval\":20,\"supportRegal\":false,\"turbo\":2,\"updateMode\":2,\"useGCForNewSurface\":false}},\"globalCSSConfig\":{\"clsName\":\"\",\"customCSS\":\"\",\"enable\":true,\"fontBold\":false,\"fontColor\":0,\"fontSize\":100},\"keyboardConfig\":{\"enable\":true,\"pageKeyMode\":1},\"networkConfig\":{\"enable\":true,\"overrideRules\":0},\"pkgName\":\"md.obsidian\",\"rotationConfig\":{\"enable\":false,\"overrideRotation\":-1},\"scrollArgs\":{\"duration\":600,\"endXPercent\":0.5,\"endYPercent\":0.8,\"sampleTime\":16,\"startXPercent\":0.5,\"startYPercent\":0.2},\"supportEAC\":true}" --id onyx_config