These instructions are idiot proof - if you can't do it I is gonna kick your butt
1. click the background layer
2. on the left toolbar double click the forground colour (the top one of the two little squares) and the colour picker opens.
3. there are two columns of numbers, at the very bottom of the left column is the rgb colour, type in the number you want
4. back to the left hand tool bar and click the paint bucket (you may need to right click the gradient symbol and choose paint bucket
5. go to background click your mouse and voila, you have the background filled
if you want to do anything fancy like change opacity then do that before you use fill colour, they are all set out on the top toolbar when you click on the paint bucket
easy peasy in'it
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Looking at your screenshot, I can see that most of your pallets are minimised and some buttons are not displayed. Not really good if you are learning to use the program.
Your toolbar is on your right. Most users keep it to the left and then the pallets on the right. Easier to use this way imo.
Already filled you in with what Sally said above. The paint bucket (fill tool) is hidden behind the gradient button. Either hold left or right click the gradient button and then you will reveal the fill tool.
Make sure you are on the background layer (or the desired layer you want to fill) and the colour selection set to the foreground. Click the area and it should fill up nicely.