MythTV themes are designed so that their elements will be properly aligned at 100x100 DPI (dots per inch). Therefore, you must specify a DisplaySize that results in 100x100 DPI if your display isn't actually using 100 DPI. To make this very clear, it does not matter what resolution/size your display actually is--MythTV depends on the DPI being 100x100 in order to draw the UI correctly.
X will normally attempt to probe the monitor during initialization in an attempt to find out it's actual size using EDID, but some monitors respond with incorrect information, and televisions don't respond to EDID queries at all, making it sometimes necessary to manually set this information in your X config file using the DisplaySize directive. If xdpyinfo reports that your display is using something other than 100x100 DPI, you can use the DisplaySize directive to fix it.
Alter the "Monitor" declarations for each display you're going to be using for MythTV in your X configuration file (XF86Config or xorg.conf) to contain a DisplaySize entry like below:
Section "Monitor" Identifier "Television" VendorName "Wombatronic Industries" ModelName "Colormeister 5000" HorizSync 30 - 50 VertRefresh 60 Option "DPMS" # For 1920x1080 at 100dpi (16:9) # DisplaySize 487 274 # For 1280x720 at 100dpi (16:9) # DisplaySize 325 182 # For 1280x960 at 100dpi (4:3) # DisplaySize 325 243 # For 1024x768 at 100dpi (4:3) # DisplaySize 260 195 # For 800x600 at 100dpi (4:3) # DisplaySize 203 153 # For 640x480 at 100dpi (4:3) # DisplaySize 162 121 EndSection
Uncomment (remove the pound symbol/hash at the beginning of the line) the DisplaySize line matches the resolution you're going to be using on that display. See also the FAQ for more details and for information about setting DPI when using NVIDIA drivers.
If your display is using a resolution not listed above, it's very easy to compute the values you need for DisplaySize. Use "DisplaySize x y" where x = (horizontal resolution)*0.254 and y = (vertical resolution)*0.254, both rounded to the nearest integer. For 1024x768, this would be x=1024*0.254, or x=260, and y=768*0.254, or y=195, so you would use "DisplaySize 260 195".