To use a well worn cliche - if you wanted to go there, then you wouldn't have started from here!
Using a spirit stain alone on pine is only going to give you a thin and dirty looking colour.
Better to start again by sanding the pine wood first before making the frame.
Use 180grit first to flatten the wood, then finish with 320grit.
Now make the frame. You can tidy the mitres then with 320grit again.
Choose a wood stain from the selection here -
https://www.ebay.co.uk/itm/32527106...2ZBMXJ&hash=item4bbbaaa053:g:PVkAAOSwVxpbT7c9
These are water-based stains and much easier to use. Slosh it on, leave it for a few minutes, then wipe it off with a rag.
If you need it darker, do another coat. You can dilute these with water for a lighter colour.
Adding to all this, buy some Van Dyke crystals -
https://www.ebay.co.uk/itm/18601732...0dbb:g:O2AAAOSwTo1kyiYb&itmprp=enc:AQAJAAAA4E
These dissolve easily in hot water. Make up a cup full to a strong solution, and add a little to the wood stain to give it a pleasing 'tone'.
When the frame has dried, give it a quick coat with this sanding sealer -
https://www.ebay.co.uk/itm/32555324...-a3f7-96a787f26106|iid:1|vlpname:vlp_homepage
Dilute some 50:50 with meths and wipe it on with a clean cloth, folded into a pad.
To give the frame colour a bit of 'body' you need to add some brown earth pigments. These are called earth pigments because they mimic the natural colours of nature and wood.
Choose Van Dyke Brown and Raw Umber -
https://www.jacksonsart.com/winsor-newton-galeria-acrylic
Squeeze out 2" of Van Dyke Brown and 1" of Raw Umber.
Mix them together by adding some water, then dilute further until you have a TRANSLUCENT glaze.
Use a 1" wide flat watercolour brush.
You have to make your own judgement here. The glaze should not obliterate the stain colour but add a certain solidity to the depth of finish.
After that has dried, paint on TWO coats of the diluted sanding sealer using the watercolour brush and leave to dry overnight.
Next day, sand the surface lightly with 600grit paper. Sanding will kick up a white powder from the sealer. This is fine but make sure not to dig too deep as this will disturb the underlying earth colour glaze. The sand paper should only have a white deposit on its surface. If you see brown, you're going too deep.
Wipe away the frame surface with a cloth and rub on some wax and you're done!