Keeping up or lapsing skills? I very much understand. I'm in the same situation.
Let's go through this one at a time:
a) line number background/foreground colours
not sure. I had thought it might be available under menu:settings->editor->"Syntax Highlighting"
but there doesn't appear to be an entry. Will dig.
EDIT: found it
menu settings->Environment->Colours-> select "Editor" from the drop down selection -> line number colouring is in this small group (Line Number foreground/background colours)
b) font
menu settings->editor ->"General Settings" dialog
top shows the editor font with a sample of how it is presented.
On my system, I have the default set to "Monospace" -> if you click the "Choose" button you can change the font.
Note: this is system dependent. If you don't see a monospaced font - add these fonts as per your system.
As for previous settings, not sure - again system dependent.
c) indents
same dialog as above -> group box below fonts labelled "Indents"
there is also the settings->editor ->"Source Formatter"->Indentation tab