I edited it for you 
but the easiest way is actually to just first put three backticks ``` on a new line in the editor.
then paste your code below it and then end the code with another ``` on a new line.
Anyway. Here is my suggestions for your code. 
note: your mouse might not line up with what you are seeing as the drawing is happening at a larger scale. you would need to do some mapping of the mouse to have it draw right under your cursor, but maybe it is fine without?
//-----------------------------------------------------
void ofApp::setup(){
ofBackground(0);
//set up the FBO for the picture
//first of all, allocate a large bit of space for the FBO. We are trying to get plenty of space for the drawing so we can print high resolution
_fbo.allocate(4000,4000, GL_RGB);
//Clear any junk from the FBO
_fbo.begin();
ofClear(0);
_fbo.end();
//Set up the Gui
myGui.setup();
myGuiHide=false;
myGui.add(color1HueMultiply.set("HueOne*",0.01, 1.0, 0));
myGui.add(color2HueMultiply.set("HueTwo*",0.01, 1.0, 0));
myGui.add(paramSettingsFileName.set("Settings file", "settings.json"));
myGui.add(buttonSaveSettings.setup("Save settings"));
myGui.add(buttonLoadSettings.setup("Load settings"));
// Setup listeners for parameters
buttonSaveSettings.addListener(this, &ofApp::saveSettingsButtonPressed);
buttonLoadSettings.addListener(this, &ofApp::loadSettingsButtonPressed);
}
//-----------------------------------------------------
void ofApp::update(){
}
//----------------------------------------------------
void ofApp::draw(){
ofBackground(10,10,10,255);
//ofSetBackgroundAuto(false); //<------ comment this out
_fbo.begin();
//draw the cirle pattern inside the fbo every frame
pattern();
_fbo.end();
//add the code for saving to the FBO
if (_saveToFbo) {
_saveToFbo = false;
ofPixels pixels;
_fbo.readToPixels(pixels);
ofImage image;
image.setUseTexture(false); //no need for a texture as we are just saving
image.allocate(_fbo.getWidth(), _fbo.getHeight(), OF_IMAGE_COLOR);
image.setFromPixels(pixels);
string filename = "circles_" + timeStamp() + "_" + ofToString(saveCount,3,'0') + ".png";
image.save(filename);
}
//now draw the fbo to screen fitting the fbo into the window viewport
ofRectangle fitToScreen(0, 0, _fbo.getWidth(), fbo.getHeight());
fitToScreen.scaleTo(ofRectangle(0, 0, ofGetWidth(), ofGetHeight()), ofAspectRatioMode::OF_ASPECT_RATIO_KEEP, ofAlignHorz::OF_ALIGN_HORZ_CENTER, ofAlignVert::OF_ALIGN_VERT_CENTER);
ofSetColor(255);
_fbo.draw(fitToScreen);
//draw the gui over everything - this won't affect the fbo contents
if( !myGuiHide ){
myGui.draw();
}
}
//-----------------------------------------------------
void ofApp::pattern(){
//declare local color var
float C1 = mouseX;
float C2 = mouseY;
ofColor color1;
color1.setHue(mouseX * color1HueMultiply);
color1.setBrightness(ofMap(mouseX,110, ofGetWidth(), 0, 100));
color1.setSaturation(ofMap(mouseX, 100, ofGetWidth(), 0, 255));
ofColor color2;
color2.setHue(mouseY*color2HueMultiply);
color2.setSaturation(ofMap(mouseY, 0,ofGetHeight(), 0, 30));
color2.setBrightness(ofMap(mouseY, 0, ofGetHeight(), 0, 155));
ofColor color3;
color3.setHue(mouseX/2);
color3.setSaturation(ofMap(C1/C2, 0,ofGetWidth(), 0, 100));
color3.setBrightness(ofMap(mouseY, 0, ofGetHeight(), 0, 255));
float nPosX = posX + ofRandom(3,5);
float nPosY = posY + ofRandom(3,15);
float wander = ofRandom(5);
ofSetCircleResolution(200);
ofSetLineWidth(ofRandom(0.1,2));
ofSetColor(color3);
ofNoFill();
ofDrawEllipse(posX, posY, nPosX/2+wander, nPosY/2+wander);
ofSetLineWidth(ofRandom(0.1,3));
ofDrawEllipse(posX, posY, nPosX/2, nPosY/2);
ofSetLineWidth(ofRandom(0.4,2.5));
ofDrawEllipse(posX, posY, nPosX/4, nPosY/4);
//update values for posX, posY
posX = nPosX;
posY = nPosY;
}
//-----------------------------------------------------
void ofApp::keyPressed(int key){
if( key == 'h' ){
myGuiHide = !myGuiHide;
}
if (key == 'c') {
_fbo.begin();
ofClear(0, 255);
_fbo.end();
}
if (key == ' ') {
_saveToFbo = true;
}
}
//-----------------------------------------------------
void ofApp::mousePressed(int x, int y, int button){
posX = mouseX;
posY = mouseY;
}
//-----------------------------------------------------
void ofApp::saveSettingsButtonPressed() {
// Get the fileName from the GUI parameter
string fileName = paramSettingsFileName.toString();
myGui.saveToFile(fileName);
}
//-----------------------------------------------------
void ofApp::loadSettingsButtonPressed() {
// Get the fileName from the GUI parameter
string fileName = paramSettingsFileName.toString();
myGui.loadFromFile(fileName);
}
