It depends on the grade you teach, but when I think BTS, I think of apples. It also fits in with September and Johnny Appleseed. I did my room in stars and have kept it up all year. I am planning on a castle/knight theme next year. You could also archive various themes and see which one suits your taste and budget. For my stars, I have stars hanging from the ceiling. I also have some stars on my door and cabinets. I have star borders and have made the headings on my b-board something involving stars. I have one of those 9 or 10 feet posters about stars. My Math Wizards board has stars. It was something simple for me to plan and fit my budget. Our school's mascot is stars as well. I just didn't keep it limited to patriotic colors. My word wall has large stars with the letters and smaller stars with the words on it.
I prepared a nuatical theme in my class last year and both my students and parents loved it! It is a fitting theme since many of us are back to work in August! I used a real large fishing net. The natural thick cabled kind and they can easily be found at either Michael's or Joanne's. I made one large fish tempate and cut out the rest in various colors so that each child was unique. Wrote their names on them for open house and when they came in took individual photos of my new students so that I could quickly affix their faces on them soon after. I then assembled them inside the net and also place shells and star fish and other nautical decorations around the net. I used beading string (dental floss or fishing wire would work too) so that I could avoid using glue and re-use. Just tie them around the net. The wording was " What a Great Catch!" Very Cute!